Removal and recovery of phosphonates from wastewater via adsorption

Abstract Over the past decades, the increasing consumption of phosphonates, which are limited resources, has attracted greater attention at the world level. Phosphonate in wastewater is a crucial resource while it may cause eutrophication as a serious environmental problem. Hence phosphonates removal and recovery from wastewater are crucial for sustainable development consideration. Adsorption as a cost-effective technology for low-concentration phosphonates resource utilization has contributed to sustainable wastewater treatment practices. Nevertheless, a systematic review on phosphonates removal and recovery from wastewater via economically realistic adsorption technique is still missing and therefore is realized in this review, especially regarding novel adsorbents. Additionally, we discussed the influence of pH and metal ions on adsorption performance and the role of characterization and chemical computation techniques for phosphonates removal mechanism. The recently established novel adsorbents (granular ferric hydroxide, ZnFeZr@MP, La/Zn/Fe3O4@PAC, Zr-La@Fe3O4, and montmorillonite) provided good adsorption capacity and removal efficiency of phosphonates. Specifically, the ZnFeZr@MP was implemented for practical application in industrial and municipal wastewater, and ZnFeZr-oxyhydroxide suggested a potential endothermic process could be realized in application practices. High desorption efficiency typically obtained by ZnFeZr (NTMP-90%; DTPMP-100%), Zr-La@Fe3O4 (96.1% and 101.8%), and clay minerals (100%) from the reusability tests. During the practically column breakthrough treatment >90% removal efficiency and 78% desorption efficiency of NTMP were achieved from membrane concentrate. Further, various modern characterization tools enhanced the investigation of phosphonate removal mechanisms. Thus, this review article is recommended to explore the mechanism for phosphonate removal and adsorption via characterization and chemical computation techniques in the future. Graphical abstract


Characteristics of phosphonates
Phosphorus (P) is a finite resource and essential element for agriculture and food production but so far, it is only considered to be recovered in a few countries in the world. Among the various types of phosphorus, organophosphonates represent an important kind of molecules because of their effectiveness as pesticides and nerve agents (Jang et al., 2015). Phosphonates are organophosphorus compounds that comprised with carbon-phosphorus (C-P) bonds. The bond of C-P is hydrolytically stable and chemically inert (Kamat & Raushel, 2013).
Phosphonates are strongly polar compounds that appear as one or more phosphonic acid groups [-C-PO(OH) 2 ] or [C-PO(OR) 2 (R ¼ alkyl, aryl)]. Phosphonates are particularly H 2 O soluble, and their ability to elevate chemical stability makes them useful in industries (Wang et al., 2019). Additionally, phosphonates display high adsorption on the mineral surfaces due to their high polarity and negatively charged groups . Phosphonates have low volatility and poor solubility in organic solvents (Horold, 2014).
Phosphonate species contained with more than one group are called polyphosphonates. Various quantitative phosphonate species (2-Phosphono-butane-1,2,4-tricarboxylic acid: PBTC, 1-Hydroxyethylidene-(1,1-diphosphonic acid): HEDP, Nitrilotrismethyl-phosphonic acid: NTMP, Ethylenediaminetetra-(methylene phosphonic acid): EDTMP, and Diethylenetriaminepenta-(methylene phosphonic acid): DTPMP); structural formula and functional groups were illustrated in Table S1. The reasons of low P content for PBTC are: (1) PBTC has a larger molar mass than HEDP; (2) high molar mass of this phosphonate species for only 11.5% of P content in the molecule (comparatively major reason); (3) contains only one phosphonic acid group; and (4) chemical structure is very different than polyphosphonates . For the strong base group (NTMP), the pKa values were varied in the range of <0, <0, 1.50, 4.62, 5.90, 7.25, and 14.2, and NTMP is one of the strongest base polyprotic acids which constituted with six ionizable -POH groups (Tribe et al., 2006;Grossmann et al., 2004). Thus, NTMP with an increase of pH switched to a negative charge (À3 to À5) (Mart ınez & Farrell, 2017).The estimation of stability constants was obtained in the relevant study for some phosphonic acids and phosphoric (Rott, Nouri, et al., 2018).The acid dissociation constants were helpful for calculating the shares of each species for the sum curves modeled of adsorption. Phosphonates contributed as particulate P fraction (PP) and dissolved unreactive P fraction (DUP) as shown in Equations (1) and (2), respectively, and mostly found with larger molecular weight fraction in the dissolved form of organic P (Rott et al., 2020a). Additionally, phosphonates species (ATMP, HEDP, PBTC, EDTMP, and DTPMP) can be analyzed by anion exchange chromatography method, and electrospray ionization coupled to tandem mass spectrometry (IC-ESI-MS/MS) was developed for the detection of these phosphonates species from river water, surface water, and wastewater (Armbruster et al., 2020). Moreover, 31 Psolid-state-NMR (nuclear magnetic resonance spectroscopy) was implemented to evaluate the proportion of organic phosphonate species in the sediment samples (Rott et al., 2020b).
The DUP concentration consisted of the difference between dissolved phosphorus (DP) and orthophosphate-P which dominated through the organophosphonates (Lucci et al., 2012). In a recent study, it was demonstrated that the dissolved non-reactive phosphorus (DNRP) which took significant phosphonates proportion in wastewater treatment plant (WWTP) effluent as 10-40% in the effluent (Rott et al., 2020a). It was found that 10% of P in dissolved and particulate phosphonates form (Whitney & Lomas, 2019), while 95% of total P 95% content was found in the form of phosphonate (Miceli et al., 1987). In the effluent of European WWTPs, an increasing fraction of dissolved organic P was observed with phosphonate chelators (PCs) (Gu et al., 2011).

Phosphonates consumption and demand for recovery
Phosphorus is a major crucial element for biomass flourishing. Hence, its removal and recovery from wastewater is an important way to minimize the nutrients extent go through the aqueous conditions and consequently halt the H 2 O eutrophication and algal diversity (Rott, Steinmetz, et al., 2018). For global fertilizer demand, Asia was predicted to account for 40% of the fertilizer forecast of 223.1 million Mt in 2030 (Tenkorang & Lowenberg-DeBoer, 2009). The long term use of organic fertilizer (mainly phosphorus) reduced soil acidification which promoted to increase in plant growth (Lin et al., 2019). The phosphorus removal efficiency relies on the constitution with particular of different types of phosphorus species existing in wastewater. Phosphonates have been recognized as recalcitrant pollutants in wastewater with increasingly recognized as a paramount resource. Phosphonates are widely used in cooling plants, paper, and textile industries, cleaning agents, household detergents, and reverse osmosis (RO) desalination plants (Boels et al., 2012). The progress of phosphonates usage in detergents, care, and cleaning (DCC) products from 1988 to 2018 in Germany is shown in Fig S1 and discussed in section S1 (Rott et al., 2020a). The long time usage of phosphonate was 2000 t (1989) and 4673 t (2015) in maintenance materials, cleaning, and detergents (Armbruster et al., 2020). Table S2 Illustrates the phosphonates consumption at different countries' levels. The modern agriculture, food manufacturing, global population, and fertilizers industries were over-relied on P because it is an important resource. By 2050 the P demand was estimated to be increased approximately 50-100% (Cordell et al., 2009). Moreover, when phytoplankton goes down to sediment and becomes deteriorated, nutrients will be released. With oxygen deficiency and the increased respiratory rate of bacteria, toxic compounds like ammonia and hydrogen sulfide will be released with increase of anaerobic micro-organisms movement. For eutrophication and plant growth, P is the minimal factor. Assimilation of orthophosphate from P could only be done by autotrophic organisms (Correll, 1998). It is essential to remove and recover phosphates from wastewater from the viewpoint of environmental protection and resource recovery (Lei et al., 2020). Hence, phosphonates removal and recovery become vital for numerous reasons. First, phosphorus is a finite resource and will soon be exhausted if no recovery is realized; Second, accumulation of phosphonates in the river sediments and suspended solids with unsettled endure consequences eventually cause eutrophication and promote environmental phosphorus loading problem; Third, when phosphonates are discharged into the environment such as water bodies that promoted to total P, and orthophosphate formation. Simultaneously, large production of different kinds of algal and cyanobacteria species causes health issues for aquatic life; Fourth, phosphonates lead to metal catalysis oxidation, degradation of microbial diversity, and hydrolysis because of the natural elimination mechanism in water (Armbruster et al., 2020;Sillanp€ a€ a et al., 2011;Saidani et al., 2021); Fifthly, chemical costs can be reduced by recovery and reuse of phosphonates (Rott et al., 2020b). Overall, it is important to remove and recover of phosphonates from wastewater.

Phosphonates in various types of wastewater
Phosphorus in the various forms of dissolved P in natural water and wastewater included inorganic orthophosphate, organic phosphates (aminophosphonates, organic phosphorus pesticides, inositol phosphates, phospholipids, and phosphoamides), and polyphosphates (Gimbert et al., 2007). In general, very limited data have been published on phosphonates quantification because it is difficult to obtain the concentration of phosphonates due to the lacking of chromophores and slow reactivity (Studnik et al., 2015;Schmidt et al., 2014;F€ urhacker et al., 2005). More details on phosphonates concentration in wastewater are presented in section S2.

Study objectives
Over the years, various technologies have been researched for the removal of phosphonates, for example, photolytically (UV/Fe II) method , Photo-Fenton, and flocculation . During the Fenton processes, a high amount of sludge was typically produced when the Fenton reagent was treated at low pH. The flocculation method could be applied at the cost of a high amount of metals used for phosphonate removal and the control of optimum flocculant dosage was difficult due to the complex matrix of wastewater (Rott, Minke, Bali, et al., 2017). On the other hand, the biological treatment of wastewater was usually inefficient for phosphonates removal due to biological degradation resistance (Rott, Steinmetz, et al., 2018). Alternatively, adsorption seems to be a promising technique for phosphonate removal and recovery from wastewater due to its greater efficiency and simple operation (Rott, Minke, Bali, et al., 2017). Nevertheless, to the best of our knowledge, until now there has been no review study published regarding the adsorption technology for the removal and recovery of phosphonates. Few review studies were performed on the removal and biodegradability of phosphonates from wastewater treatment plants, with a minor part of phosphonate adsorption included (Rott, Steinmetz, et al., 2018). The economically and environmentally powerful adsorption technique for developing nanomaterial's, phosphonates as recovered for agricultural application, and nano based fertilizers have been significantly established recently Rakhimol et al., 2021). The reusability is obligatory to recover the adsorbent value and prevent secondary pollution .
Overall, the objectives of this review article were aiming to discuss: (1) the adsorption process and the adsorptive removal of phosphonates from wastewater; (2) the enhanced phosphonates adsorption by novel adsorbents via various metals, and the role of pH for phosphonate adsorptive removal; (3) the regeneration of the adsorbents for phosphonates removal; (4) the adsorption mechanism for phosphonates removal via various characterization techniques and chemical computation techniques.

Adsorption technique for phosphonates removal
Adsorption is traditionally proposed as a polishing technique for wastewater treatment but could be economically realistic for practice; in fact, adsorbents are imperative to be especially low cost or environmentally friendly (Li et al., 2016;Loganathan et al., 2014). Specifically, adsorption for phosphorus removal has attracted a lot of attention owing to the relatively facile implementation, high efficiency, and reusability Lin et al., 2020). Especially, adsorption is a technique that has frequently been investigated to remove low concentrations of P (Sengupta & Pandit, 2011;Awual et al., 2014). Adsorption is usually applied for orthophosphate (o-P) removal but studies for the removal of phosphonates via adsorption were also reported (Kumar et al., 2010;Boels et al., 2012). Conversion of phosphonates into phosphate by pretreatment such as the advanced oxidation method can be an alternative option followed by adsorption (Mayer et al., 2013), the treatment cost may be however increased accordingly.

Overview of adsorbents
Adsorption behavior depends on the surface properties and structure buildings of the applied adsorbents. The different adsorbent types are usually categorized into two types. One type is natural adsorbents such as wastewater sludge, coconut shell charcoal, activated carbon, activated coconut shell charcoal, sediments, mussel shell, powdered laterite stone, and soils (Kumar et al., 2010). Adsorbents including fly ash (Li et al., 2016) or modified agriculture waste/by-product adsorbent generally exhibited high adsorption capacity but poor recyclability could be responsible for the limited applications . The other type is synthesized adsorbent. The synthesized adsorbents have also been investigated for their positive and negative effects on agriculture and the environment (Altaf, Zafar, et al., 2021). The further overview of adsorbents is illustrated in section S3. Table S3 listed relevant data for comparison among different studies, especially removal efficiency, adsorption capacity, pH, kinetics, mechanism, and experimental conditions regarding phosphonates adsorption with various adsorbents.

Novel adsorbents (a) Zinc-Iron-Zirconium (ZnFeZr@MP)
Drenkova-Tuhtan et al. (2021) examined the phosphonate removal with pH function and adsorbent dosage from a synthetic solution of ZnFeZr adsorbent, for a better fundamental perception of phosphonates removal from RO concentrates. The electrostatic interaction and OH groups coordinated on the adsorbent enhanced the removal of NTMP. The fixed dosage of 0.03 g/L ZnFeZr within pH 4-12 was attributed to the adsorption efficiency in the followed order: HEDP > EDTMP > NTMP. This implied the mechanism of electrostatic repulsion between the charge of adsorbent and phosphonates charged with increasing pH. A neutral pH 7, initial concentration of 1 mg/L, and 0.1 g/L of ZnFeZr@MP were suggested to be considered for practical application in industrial wastewater effluents or municipal wastewater.
(b) Lanthanum-Zinc-Iron oxide/powder activated carbon magnetic (La/Zn/Fe 3 O 4 @PAC) Some studies (e.g. Greenlee et al., 2010) demonstrated the existence of antiscalants could hinder the precipitation of dissolved salts in RO concentrates, and thus removing phosphonates from RO concentrates has been a particularly urgent need. For example, Li et al. (2021) investigated the removal and mechanism for HEDP antiscalant from RO concentrate with low cost magnetite La/Zn/Fe 3 O 4 @PAC. The removal efficiency of HEDP increased with increasing adsorbent dosage, while adsorption capacity was decreased at higher dosage due to ineffective adsorptive sites. The HEDP adsorption process was well described through pseudo second order kinetic model with a mechanism of promoted chemisorption.
(c) Fe 3 O 4 nanoparticles coating with poly dimethyl diallyl ammonium chloride (Fe 3 O 4 /PDDA) The use of magnetic adsorbents for removing pollutants and resource recovery is an increasing research field in wastewater treatment. For example, Chen et al. (2019) investigated the removal of HEDP (36 mg/L initial concentration) by innovative magnetite nanoparticles Fe 3 O 4 /PDDA (Fe 3 O 4 coated with poly dimethyl diallyl ammonium chloride) with 1 g/L dosage at pH11. The pseudo second order and freundlich model is well described for that adsorbent and HEDP removal. Especially, the adsorbent could maintain high adsorption capability for wastewater treatment containing phosphonate. Because quaternary ammonium groups existed on the Fe 3 O 4 / PDDA surface that enhanced the HEDP adsorption. Fe 3 O 4 /PDDA magnetic adsorbent has the potential for organic phosphonate removal from wastewater as an environmentally friendly magnetite adsorbent.
(d) Zirconium-lanthanum modified magnetite (Zr-La@Fe 3 O 4 ) Altaf, Lin, Tadda, et al. (2021) conducted an adsorption isotherm experiment. They obtained higher NTMP removal at acidic pH of 4 under low initial NTMP concentration by using novel magnetite Zr-La@Fe 3 O 4 . Langmuir isotherm model suggested monolayer mechanism while kinetics study indicated that pseudo second order enhanced chemisorption and adsorption kinetics analyzed by Equations (3) and (4). At the higher dosage of Zr-La@Fe 3 O 4 , the removal efficiency was greater owing to more available adsorbent sites. Also, removal efficiency decreased with increasing pH conditions from pH 3 to 8, contributing to electrostatic repulsive forces. The high removal efficiency of 99% with 2 mg/L NTMP suggested the good potential of treating real wastewater with Zr-La@Fe 3 O 4 in the future. Pseudo first order model Pseudo second order model (e) Clay minerals (montmorillonite (Mt) and Kaol) Zhu et al. (2021) investigated batch adsorption for 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTC) with two different kinds of nontoxic clay minerals (Mt, and Kaol) and studied the systematical kinetic and isotherm adsorption which is coincided the pseudo second order model (Chemisorption process), and Langmuir model (monolayer interaction) respectively. The higher removal efficiency (92%) and adsorption capacity (121.163 mg/g) of PBTC were obtained at lower pH 3 and 1 g/L of adsorbent dosage using Mt than kaol. The thermodynamic findings estimated by Equations (5)-(7) indicated the removal behavior was endothermic, and spontaneous for the Mt mineral. This adsorption study uses these minerals to explore the further interface of different clay minerals.
2.2.2. Other various adsorbents (a) Granular ferric hydroxide (GFH) Reinhardt, Elordi, et al. (2020) reported higher adsorption loading (adsorption capacity) of NTMP at lower pH which showed a positive net charge when pH was below the point of zero charge (pH PZC ), and the interaction between GFH and NTMP was stronger electrostatically at lower pH. In terms of thermodynamic effect, they found that adsorbent response was poor at low temperature, suggesting low wastewater temperature for phosphonates adsorption on GFH should be avoided in technical applications. Additionally, Isotherm equilibrium was not obtained after three days, and empirically equilibrium conclusions regarding the mechanism of adsorption should be estimated carefully. Therefore, isotherm equilibrium time was obtained at 168 h.
Further, Chen et al. (2017a) demonstrated isotherm adsorption on GFH for NTMP with different initial concentrations of NTMP. The best fitting of Langmuir model (R 2 ¼ 99.6%) and Freundlich model (R 2 ¼ 99.8%) at 3.0 mg/L NTMP concentration were obtained, and the mechanism was inner sphere complexation involved. Additionally, they conducted a column breakthrough experiment to examine if phosphonate kinetic adsorption was adequate to be applied in a practical flow treatment system, but the adsorption kinetics was slow and not reversible.
Besides, Chen et al. (2017b) investigated the adsorption of NTMP through the column breakthrough from membrane concentrate. The NTMP adsorption with adsorbed silica residue was less favorable than direct NTMP adsorption on ferric hydroxide, where the direct adsorption process evolved due to increases in NTMP surface coverage. The chemisorption of NTMP built up the negative charge on the surface of ferric hydroxide which was less preferable. Also, the NTMP adsorption equilibrium stage was achieved within 0.03 h with a constant rate of K (1.72).
(b) Zinc-iron-zirconium (ZnFeZr) oxyhydroxide magnetic Rott, Nouri, et al. (2018) suggested magnetic harvesting makes the investment cost in filtration units or sedimentation. This option for subsequently P recovery as a fertilizer product makes the technology a better alternative to the treatment of conventional techniques. However, magnetic particles were used for the removal of organic phosphorus. The ZnFeZr-oxyhydroxide magnetic showed the best performance at 1 mg/L concentration, pH 6, and 0.06 g/L of dosage for the NTMP removal. In addition, NTMP adsorption was endothermic using magnetic adsorbent (ZnFeZr-oxyhydroxide), the process could be significantly implied in application practices such as through the utilization of waste heat during the production operation. On the other hand, Rott, Reinhardt, et al. (2018) introduced a method, as shown in Figure 1 (miniaturized P determination: ISO 6878 (ISO mini ), which permitted an investigation of phosphonates adsorption with the following points: (1) phosphonates adsorption on granulated filter materials, (2) particular granular ferric hydroxide (GFH), (3) with little effort, (4) high-level reliability, (5) cost savings, (6) concerning phosphonates little workload. The main advantage of this method is not to neglect any Figure 1. Optimized procedure for determining the phosphonates adsorption by using granular ferric hydroxide according to the modified and miniaturized pattern of ISO 6878 (ISO mini ) with screw cap vials (10 mL), buffer dependent potassium peroxodisulfate concentrations, heating in thermostat, and direct insertion of color reagents into the digested sample without transferring it first (Rott, Reinhardt, et al., 2018). accuracy due to the simplification than the standardized process. The findings showed lower adsorbent loading of NTMP gained with increasing pH, indicating protonated or deprotonated because Iron oxide has a huge number of Fe-OH groups on its surface. The higher pH values (12) lead to electrostatic repulsion, therefore a much higher concentration of NaOH must be used for successful desorption.
Based on the surface complexation model, the phosphonate adsorption mechanism was mononuclear complexes with more than two protonation levels. The chemical reaction by Equation (8) presented the adsorption stoichiometries (Nowack & Stone, 2006).
where FeOH is the surface site and L aÀ is the deprotonated form of phosphonate. The efficiently adsorbed phosphonate at pH 7 is attributed to wastewater treatment plants with different affinities of goethite surface oxides. Additionally, both Salvado et al. (1999) and Zenobi et al. (2005) implemented metal hydroxides such as a-FeOOH (goethite) and !-AlOOH (boehmite) for removal of phosphonates from wastewater and observed strong adsorption of phosphonates.
(d) Coated filtration sand, activated carbon and anion exchanger The practical application of sand coated on iron oxide is mainly observed for the adsorption of heavy metals (Benjamin et al., 1996), phosphates (Boujelben et al., 2008;Zeng et al., 2004), and humic material (Lai & Chen, 2001). However, phosphonate removal on iron coating sand from membrane concentrate does not determine yet. Further, Boels et al. (2010) investigated the NTMP removal from membrane concentrate at 25 C with NTMP concentrations of 5-50 mg dm À3 (real nanofiltration concentrate) with low cost coated filtration sand (CFS) adsorbent, and 5-300 mg dm À3 (synthetic concentrate solution) with anion exchanger amberlite (IRA-410 and IRA-900), coated filtration sand, and activated carbon. They observed higher adsorption capacity (IRA-410 ¼ 144.2 mg/g, and IRA-900 ¼ 116.2 mg/g) at lower NTMP concentration due to the positive surface charge which caused the functional groups at the equilibrium stage with ionic strength of 0.1. In real nanofiltration, CFS presented the maximum adsorption capacity of 4.05 mg/g via adsorption isotherm under pH 7.0 to 8.0 because of lower positive surface charge. In synthetic solution, adsorption capacity of 13.4 and 7.92 mg/g were observed for coated filtration sand and activated carbon respectively. Comparably, the isotherm slope showed lower capacity, indicating weak interaction with NTMP (low K ads ). This isotherm model can easily be incorporated into the future design model. Thus, amberlite (IRA-410) indicated the best performance for NTMP adsorption which is a relatively expensive material than coated filtration sand, but coated filtration sand shows a promising mean for practical application.
(e) Natural adsorbent and polypyrrole modified red mud adsorbent (PMRM) Kumar et al. (2010) studied the removal of potassium phosphonate from water by using different natural adsorbents including coconut shells charcoal (CSC), mussel shell (MS), ACSC (activated coconut shell charcoal), synthetic polymer, amberlite (IRA 400 LR), and powdered laterite stone (PLS). A reasonable removal percentage of phosphonate (40.40%) was obtained within 15 minutes, and higher adsorption capacity (8.08 mg/g) was achieved with low cost powered of natural laterite stone (PLS) than other adsorbents. Because, PLS has a high level of hydroxides and iron which is highly contributed as strong binding with phosphonate. In addition, A possible kinetics of the reaction between phosphonate and PLS was proposed as shown in Equation (9).
PLS-M-OH presented to hydroxide/oxide of iron/aluminum which existed in PLS, and a phosphonate complex was formed (PLS-M-O-PO 2 K 2 ) after the reaction. The final rate (H 2 O existed in reaction form in infinite quantity) can be represented through the concentration of PLS-M-OH and HO-PO 2 K 2 . Hence, the kinetics reaction presented the Pseudo second order. The other reaction between the phosphonate and CSC was shown in Equation (10) which revealed the CSC̶ [X y O] (alkali/alkaline earth metal) occurred on the surface of CSC. In the solution, the alkali metal was released during the oxidation of phosphonate into phosphate.
Yin et al. (2022) reported the adsorption capacity of HEDP (through batch ¼ 54.7 mg/g; through bed column ¼ 0.27 mg/g/min) showing an efficient adsorption process of HEDP on polypyrrole modified red mud adsorbent (PMRM). The fixed bed column experiment for the adsorption process of HEDP as shown in Figure 2 could be important for practical applications. In addition, rising flow velocity outstandingly enhanced the capacity of adsorption column, and models (Thomas and Yoon-Nelson) were competent for the adsorption column of HEDP. In prior to, adsorption column studies with PMRM should be more insight elaborate for phosphonate adsorption.

Fast magnetite for potential field application
The fast magnetite for potential field application is presented in sections S4 and Figure S2 (Drenkova-Tuhtan et al., 2017).

Metal ions influence on phosphonate adsorption
The metal with a well-rounded d-orbital and soft shell (lead ion: Pb 2þ ), and transition metal (copper ion: Cu II ) with fractionally filled d-orbital are greater chelated through the phosphonates as compared to the hard sphere metal (calcium ion: Ca 2þ ) accompanied by the noble gas configuration. To the configuration of inner sphere complexes, Ca 2þ metal is not much more able to displace water hydration (Benjamin, 2015). Especially, magnesium and calcium ions with high concentrations can be frequently elevated in the wastewater such as membrane concentrates (Sperlich et al., 2010;Antony et al., 2011). It is worth noting in some earlier studies, calcium showed a positive effect on phosphonates adsorption (Drenkova-Tuhtan et al., 2021;Rott, Nouri, et al., 2018). On the other hand, the phosphonate surface coverage determined by the Langmuir plot showed that in the presence of equimolar (Iron: Fe III ) or Cu the adsorption of phosphonate species was not significantly altered onto goethite. Additionally, the Isotherm NTMP adsorption with the presence and absence of Cu II and Fe III revealed that data aspects for uncomplexed Cu-NTMP and NTMP cannot be stately differentiated. In addition, the influence of Cu and Ca on the adsorption of EDTMP, NTMP, and HEDP using iron hydroxide goethite was negligible with equimolar concentrations (Nowack & Stone, 1999a). As ternary complexes for ferric hydroxide, metals and phosphonates can co-adsorb and form surface complexes such as phosphonate-Ca (Nowack & Stone, 1999a;Mart ınez & Farrell, 2017). This mechanism was preferred at lower pH, wherein solution preeminent metal species were organic and cation complexes metals while hydroxyl and carbonate complexes were enhanced at high pH (Sposito, 2016).
Further, Hein et al. (2007) studied the HEDP adsorption on alumina at pH 5-9 with Cu II metal. The study displayed the solely adsorption of HEDP, successfully the surface complexation dimensions with HEDP, and the adsorption was practically unaffected in the presence of equimolar concentration HEDP. Boels et al. (2012) indicated that phosphonate structure could be maintained by adsorption technology which can promote the reuse of phosphonates. Removing phosphonates by adsorption is a promising solution from the treatment of reverse osmosis (RO) concentrate. The Freundlich model presented a satisfactory equilibrium correlation while the 1.2 molecules/nm 2 density of NTMP did not exceed the monolayer in the presence of calcium. Therefore calcium has an advantageous role in NTMP removal on GFH.
Furthermore, Li et al. (2021) reported that Ca 2þ has a major role in the adsorption with seed crystallization which should be solved the Ca 2þ separation problem from RO concentrate in practical industrial application. Boels et al. (2010) permitted the exchangeability of experimental findings into upcoming practical applications for GFH. Moreover, suggested carbonate and sulfate effects should be determined on the adsorption of NTMP. Thus, a further effect of some other ions and real wastewater experiment should promote the phosphonate adsorption, because nanofiltration (NF) concentrate is the major source of ions. There are only a few studies have been reported for Ca influence on phosphonates adsorption with GFH from membrane concentrate (Table S3). With the aim of knowledge gaps, Reinhardt, Campero, et al. (2020) investigated the adsorption of phosphonates on GFH through the influence of Ca II which were implemented by Ca:phosphonate 0:1 and 0:2 ratio. A higher concentration of Ca II was examined regarding the removal of two salient representative species of organophosphonate (NTMP and DTPMP) which were widely used for industrial application. The results showed that the removal rate difference was less than 13% in the presence and absence of Ca II . Ca II had a generally positive effect on phosphonate removal, revealing ternary complex formation, and HEDP was created to precipitate. In addition, higher NTMP removal was achieved when the molar ratio of Ca:NTMP (18.33:1) was used at different pH conditions (Table S3). Future research should focus on phosphonate removed after multiple cycles with re-dissolved Ca II from GFH and whether the precipitate should interfere with adsorption or regeneration.

Role of pH on phosphonates removal during adsorption
The pH is an eminent quantity that displays the chemical constraints for the solution. The chemical behavior, nutrients availability, and biological functions can be controlled by pH (Brady & Weil, 2002). For the adsorption behavior of adsorbents, pH value is a significant factor for the adsorbent surface charge changes and the adsorbate's ionization level (Wang et al., 2015). When pH exceeds 8.0, extra protons become dissociated, NTMP protonation levels can occur with different changes through the pH function (Kan et al., 2005). Zeta potential is a significant characterization technique to evaluate the surface charge which could be applied to determine nanosuspension's stability (Jiang et al., 2009). When the solution's pH was greater compared to the point of zero charge of adsorbent (pHpzc ¼ 7.0), adsorbent usually showed lowered adsorption performance of phosphonate. However, wastewater such as RO concentrates generally has a pH value of around 7.0 to 8.0, it is therefore important for adsorption performance to be stable in this range of pH (Li et al., 2021). On the other hand, most recent studies on PBTC removal performance determined the influence of pH solution using clay minerals. At the equilibrium stage, no significant effect by pH solution was observed, while the surface charge of PBTC and zeta potential was affected by the pH solution. Moreover, the pH PZC was 3.56 achieved with pH solution through net surface charge (zero). The PBTC surface was positively charged at pH < pH pzc when the adsorption process was enhanced . The results from Nowack and Stone (1999b) suggested that the polyphosphonate species have a negative charge, then phosphonates molecules could be further disturbed during the adsorption, and the negative charge was transferred to the adsorbent surface.
Moreover, at pH < 13 phosphonate groups were completely deprotonated which is indicated to dissociation of hydroxyl groups in HEDP (Popov et al., 2001). Figure 3a presented the pH effect and influence of phosphonate nature on adsorption loading of GFH.The adsorption performance was decreased for all six phosphonate species (NTMP, EDTMP, DTPMP, HEDP, PBTC, and HPAA) with increased pH (Figure 3a). Additionally, adsorption affinity decreased with increasing molecular mass and PG (phosphonate groups). Thus, remarkably, decreased phosphonate adsorption though increasing molecular size presented that larger molecules containing species generally occupied huge adsorption locality. Furthermore, when polyphosphonates species adsorbed on the adsorbent which is contained with an elevated negative charge, then other phosphonate molecules were disrupted through the negative charge during adsorption. Besides, PBTC species was deprotonated at pH > 9 with no sudden drops at higher pH conditions than pH pzc for all phosphonates, which means the adsorption process has also occurred through a negative net charge on the adsorbent surface. As well as, most of the ions were present in the real wastewater, especially cooling, membrane, paper, and textile wastewater. Therefore it is very important, that a further experiment should be investigated the effect of competing anions, and real wastewater in the future, which should be contributed to adsorption of phosphonates and enable practical realizations through transferability of findings (Reinhardt, Elordi, et al., 2020). When pH was decreased compared to pH pzc , the surface groups of the adsorbent were assembled despite pH pzc , and the adsorbent surface could be positively charged (Yin et al., 2020). Therefore, it is most important to determine the pH pzc of material, and Yin et al. (2022) investigated the point of zero charge (pH pzc ) of low cost polypyrrole modified red mud adsorbent (PRM) which is designated to positively charged ( Figure 3b). As well as Figure 3b showed that pH pzc was greater in the range of 3.3, 6.6, and 7.2 for PRM-3, PRM-2, and PRM-1 respectively. Remarkably, the increased pH pzc has presented the anionic mechanism for adsorption and removal performance of HEDP with PRM adsorbent.

Overview of desorption
Desorption is the reversal of adsorption, in which the adsorbed compounds are desorbed from the surface of adsorbent (Havlik, 2008). The most important aspects of adsorption are regeneration and desorption, and desorption can be accomplished with either thermal treatment or desorbing reagents (Sorokhaibam & Ahmaruzzaman, 2014). Desorption contributes to sustainability in two ways: (1) reuse of the adsorbent, making it reusable across a variety of effective adsorption or desorption cycles, and (2) retrieval of the adsorbate (Piol et al., 2019). Coherent desorption depends on particular circumstances and interfaces among adsorbent, adsorbate, and the adsorbent substance's nature (Volesky, 2004). The desorption study of kinetics and equilibrium are dominant for understanding the desorption features from adsorbents. In addition, for the study of desorption kinetics different models can be used including Freundlich, two constant rates, Elovich, third order model, parabolic diffusion, zero order, pseudo first order, and pseudo second order (Bashiri, 2011). The desorption rate depends on the species, bonding strength on the adsorbent surfaces, temperature, and sometimes highly dependent on the atomic response from the adsorbents. Thus, desorption should be characterized by the kinetics dealing with the desorption rate (Matsushima, 2018). The desorption solutions could relieve the problems of discharge and storage of solution (Canedo-Arguelles et al., 2013).
On the other hand, the adsorbent's reusability is extremely important in practical applications. In terms of reusability, the optimal adsorbent can be completely regenerated and reused for several cycles without declining adsorption capacity. Reusability also facilitates the recovery of P and aids in economic recovery. The adsorbent's adsorption capability can be reduced during reusability processes due to various factors. Adsorbate desorption can be low, precipitation on the surface of adsorbent, degradation of active sites due to adsorbent's wear and tear, and alterations in the adsorbent properties including crystallinity or porosity during the adsorption process and regeneration (Chitrakar et al., 2006;Kumar et al., 2018). The complex matrix such as wastewater effluent can cause problems for the recyclability of adsorbents where some ions could bind to the adsorbent. As a result, the choice of regeneration process is critical to make sure suitable discharge of bound ion (Cornell & Schwertmann, 2004).

Desorbing solution
Generally, the possible desorption solution is considered as alkalis solution, saline solutions, mineral acids, and complexing agents (Bacelo et al., 2020). The use of an alkaline solution is typically required in the regeneration process to reverse the reaction (Kalaitzidou et al., 2016). Normally, for the reusability alkaline (NaOH) solutions are more favorable, and enhancing the concentration of NaOH solution offers a greater desorption rate (Cheng et al., 2009;Drenkova-Tuhtan et al., 2016;Nur et al., 2014;Wan et al., 2017;You et al., 2018). Besides, several studies indicated outstanding desorption by implementing a low concentration of NaOH solution (Gu et al., 2017;Pham et al., 2019;Zhou et al., 2018). On the other hand, chloride and sulfate solutions did not provide an appealing desorption mechanism, suggesting a poor relationship between adsorbate and different adsorbents. Surface precipitation was observed on iron oxide adsorbents from drinking water in a previous analysis, necessitating the use of an acidic solution to reuse the adsorbents (Kunaschk et al., 2015). Nonetheless, using layered double hydroxide (LDH) magnetite microparticles, an adequate desorption efficiency of 70-96% has been reported in a combined solution of NaCl and NaOH . Phosphonate desorption from humic substances and mineral surfaces was facilitated by alkaline pH (Frigge & Jackwerth, 1991;Nowack & Stone, 1999b;Nowack, 2002). However, Ca 2þ ions were found to endorse adsorption on activated sludge and mineral surfaces (Nowack, 2002). Moreover, in the absence of calcium, from iron oxide material desorption of ATMP (NTMP) was perceived at pH 12 (Nowack & Stone, 1999a). Some studies suggested that NaOH and ethylenediaminetetraacetic acid (EDTA) should be used to extract OP from sediment (Bowman & Moir, 1993;Cade-Menun & Preston, 1996). Due to the increase in negative charge, high pH enhances the solubility of phosphonic acids, and an abundance of EDTA causes calcium ions to compete for complexation.

Reusability
The magnetic microparticles with high adsorption capability could be effectively regenerated and reused from wastewater through several loading cycles, demonstrating its ease of usage, costsaving potential, and long-term reusability Mandel et al., 2013;Schneider et al., 2017). Engineered media has a wide surface area and the capacity to be regenerated for reuse. After regeneration, the recovered phosphorus species can be used for other phosphorus products or feedstock for fertilizer, reducing the need for phosphorus rock (Kurniawan et al., 2012). Desorption efficiency after the reusability experiment could be calculated according to equation (11) (Drenkova-Tuhtan et al., 2021).
Q d is the desorbed amount; Q a is the initially phosphonate adsorbed (mg/g) Drenkova-Tuhtan et al. (2021) investigated a pilot scale test for municipal wastewater, the Zn-Fe-Zr particles' effluent polishing potential and reusability were established in 55 adsorptiondesorption cycles with no reduction in output while accomplishing 90% overall efficiency. An adsorbent dosage of 1 g/L at pH$ 7 and a reaction period of 20 minutes was enough to eliminate nonreactive phosphorus species from municipal wastewater and sustain ultra-low concentrations of effluent (<0.05 mg/L P tot ) through several cycles. These monitored conditions are more suitable to treat the real wastewater for further regeneration research in the future. Besides, Chen et al. (2019) conducted a desorption and reusability experiment of four cycles under the following conditions: 180 mL of 4 g/L of HEDP solution, 4.0 mg of Fe 3 O 4 /PDDA were added in 20 mL of DI at pH 11.0. After adsorption, magnetic nanoparticles were separated from the solution, then for the collection of adsorbent 0.1 M HCl was used and magnetic was washed with DI and regenerated particles were used again for further adsorption cycle. The adsorption capacity was however decreased with the increasing of cycle. Table 1 reported the recent literature on the reusability of adsorbents for the regeneration of phosphonates species. In the study by Rott, Nouri, et al. (2018), the engineered particles (ZnFeZr) reusability in an artificial NTMP-solution and industrial wastewater (DTPMP-rich membrane concentrate) were performed with 2 mol/L NaOH over 30 cycles by adjusting the different operating conditions (dose, pH) to improve the efficiency as seen in Figure S3. Also, after 30 cycle's complete phosphonate elimination was observed under ideal conditions. In fact, reversible sorption was achieved by 1 mg/L NTMP-P in the NaOH solution. The desorption efficiency of DTPMP in industrial wastewater was high. The adsorbent dosage (0.1 g/L) was good for long term reusability of ZnFeZr-oxyhydroxide at pH 6. Industrial phosphonate-containing wastewaters abundant in calcium were demonstrated to be particularly well suited to treating the particles (Rott, Nouri, et al., 2018). While in the study by Li et al. (2021), stability of La/Zn/Fe 3 O 4 @PAC was investigated after regeneration cycles. Based on the substitution of adsorbed HEDP by the OH-, La/Zn/Fe 3 O 4 @PAC was regenerated with 2 mol/L NaOH solution. The adsorption capability was reduced from 35.160 to 24.920 mg/g, but there was a little effect of alkali treatment on the adsorption efficiency of adsorbent. Since the adsorbent could be easily isolated and retrieved after four successive desorption cycles at pH 8 with 70% desorption efficiency, the La/ Zn/Fe 3 O 4 @PAC adsorbent was robust and reusable for HEDP desorption and adsorption. But, Boels et al. (2012) determined the regeneration performance of GFH adsorbent by using 0.1 mol/ L NaOH with a maximum adsorbed amount of 8 g kg À1 with 2.6 g GFH at pH 1. As a result, GFH can be reused, and stabilizing chloride ions can be replaced through hydroxide ions, particularly during regeneration. Ultimately, future studies should consider the risk of GFH structure breakdown and dissolution during regeneration. Nonetheless, after regeneration with an alkaline solution, it can be inferred that GFH is reusable, implying NTMP could be extracted from the RO concentrate.
On the other hand, from the study of Reinhardt, Elordi, et al. (2020), five consecutive regeneration cycles revealed high adsorption, NTMP elimination, and desorption performance at 20 C, 1 mol/L NaOH, and pH 6. There was 38% of the material (GFH) waste, and >90% removal efficiency was achieved. It was suggested that it must be checked with actual wastewater to see whether other ions have a competitive or supportive effect and in column experiments. Future studies could also look at the possibility of multiple re-uses of regeneration solutions several times. Furthermore, Chen et al. (2017a) recommended numerous competing factors must be reduced during adsorbent regeneration, including (1) the mass of base used to produce regenerated solution; (2) time needed for regeneration, and 3) the volume of the regenerating solution. The reaction rate of the desorption process, as well as diffusional mass transfer, are kinetic constraints on desorption. The NTMP discharge rate increased from 4.40 Â 10 À4 to 1.82 Â 10 À3 mg-NTMP/min by 4.1 desorption factors (this is presented that desorption efficiency was limited due to equilibrium feature). The desorption rate increased from 1.21 Â 10 À3 to 6.55 Â 10 À3 mg-NTMP/min with a desorption factor of 5.4, indicating an equilibrium effect. The discharge of phosphonate from the adsorbent is a sluggish process at a pH of 8.3 in the adsorbent reuse (0.1 mol/L NaOH) experiment by column breakthrough. The regeneration was poor for uptake of NTMP. Because the adsorbent failed to regenerate efficiently, it was unlikely to be used for costeffective antiscalant removal from membrane concentrate. For GFH to be considered a suitable adsorbent for phosphonate, further research on improving regeneration is needed. While Chen et al. (2017b) observed the regeneration response of ferric hydroxide by 0.1 mol/L NaOH solution at pH (12.7). The findings were included by > 90% removal efficiency of NTMP obtained on the regenerated material via column experiment, and 40% reduction from 4.55 to 2.71 mg NTMP/g was observed during NTMP adsorption on the regenerated substance. This suggested that desorption of the bidentate binuclear complex might be less affected by the high pH solution than desorption of the monodentate complex. Because of the incomplete regeneration, a disposable ferric hydroxide adsorbent including FeCl 3 coagulation/precipitation, was suggested to be a better choice for extracting NTMP from membrane concentrate solutions. Moreover, Altaf, Lin, Tadda, et al. (2021) demonstrated the reusability of modified magnetite adsorbent (Zr-La@Fe 3 O 4 ) with 25 mL of 1 mol/L NaOH solution. Reusability cycles 1-5 yielded adsorption capacities of 7.50, 6.39, 6.55, 7.84, and 8.61 mg/g, respectively. In the future, additional consideration is required to achieve full NTMP recovery as well as agricultural wastewater should be treated with Zr-La@Fe 3 O 4 magnetite. From the reference of , the regeneration study of Kaol and Mt was conducted by 0.1 M NaOH solution for 12 hours with seven cycles. The desorption of PBTC was enhanced from the anions and negatively charged under the efficient basic condition. After seven cycles of reusability, the desorption efficiency of PBTC decreased, and the chemical constancy of these minerals is more promising for reusability cycles. In last, Altaf, Lin, Zhuang, et al. (2021) suggested a lower desorption efficiency of NTMP due to possible conversion of NTMP into OP using Zr-La@Fe 3 O 4 with alkaline solution (NaOH) of 1.5 mol/L. Also, the removal efficiency was retained during reusability of five cycles by 84% to 93% through the NaOH solution of 1.5 and 3 mol/L. While mixture (NaOH þ NaCl) solution presented higher desorption efficiency without declined of adsorption capacity and maintained the removal efficiency 100%. The higher NTMP desorption efficiency through 0.30 mol/L mol/L NaOH þ NaCl and 3 mol/L NaOH could be effective for the reusability Zr-La@Fe 3 O 4 . Thus finally conclusively, alkaline reagents were promising for phosphonates desorption and reusability of mentioned different adsorbents as well as that adsorbents have a potential implementation for wastewater treatment in the future.

Mechanism study via characterization
The characterization techniques refer to the structure, composition, and many other properties such as magnetic, physical, electrical, and chemical behavior. The development of characterization techniques is helpful to explain the mechanism between adsorbate and adsorbent.

X-ray diffraction (XRD)
As example, the XRD spectra of Kaolinite (Kaol) and montmorillonite (Mt) with standard International Center Diffraction Data (ICDD) reference pattern for PBTC adsorption mechanism were shown in Figure 4a (Zenobi et al., 2010). For Mt, broad reflection (001) was found in 6.204 Å and at 12.309 Å sharp reflection of Kaol was shown which observed during the adsorption process crystal structure was not damaged. The majority of PBTC was adsorbed through the entrance in the interlayer . The crystallinity structure of Zr-La@Fe 3 O 4 magnetite before and after NTMP adsorption indicated to hexagonal Zr and La phase and five compounds were formed (La (OH) 3 , Fe 3 O 4 , Hematite Fe 2 O 3 , ZrO 2 , Maghemite Fe 2 O 3 ) and after adsorption amorphous structure obtained. Additionally, Rietveld refinement parameters of Zr-La@Fe 3 O 4, cell parameters, atomic positions, and space groups of phases were estimated suggesting that magnetite adsorbent was much attraction for the removal of NTMP from the wastewater because Zr-La@Fe 3 O 4 has good crystallinity. The goodness of fit was also calculated by Equation (12) for the Rietveld refinement pattern. R wp and R exp referred to weighted and expected profile R factors, respectively (Altaf, Lin, Tadda, et al., 2021). Zhu et al. (2021) investigated the FTIR spectra of Mt, Kaol, and PBTC stretching vibration O-H at 3578, 1410, and 1701 cm À1 and could describe the carbonyl group, which played an important role during the adsorption process. The bands of 1189 cm À1 obtained the -P ¼ O. These peaks suggested that the framework composition of Kaol and Mt were not collapsed through PBTC (Figure 4b). Especially, on the adsorbents, for phosphonate molecules, oxygen atom could be used as an acceptor of hydrogen bonding which made the intramolecular bonding of hydrogen with hydroxyl groups. The snapshots (Figure 4c, d) of the equilibrium adsorption indicated that PBTC on the surface of minerals could be freely adsorbed, and the carboxyl groups attributed to electrostatic interaction. The dispersibility and water solubility of PBTC was good due to the consistent distribution of water molecules. Peters et al. (2016) and Kim et al. (2020) also obtained similar findings for adsorbed phosphonate on numerous adsorbent surfaces. Altaf, Lin, Tadda, et al. (2021) recommended that FTIR analysis presented the interaction between the iron and carboxylic groups, contributing to NTMP adsorption from Zr-La@Fe 3 O 4. Zenobi et al. (2010) studied the ATR-FTIR at various pH, both in solution and adsorbed states for HEDP and NTMP. The experiment was conducted at 900-1200 cm À1 and measured with deuterium triglycine sulfate (DTGS) detector (FTIR instrument Nicolet Mangna 560). The 4 cm À1 of spectral resolution, operational temperature was 25 ± 2 C, and the spectrum was derived by 256 co-added interferograms. The observation of IR bands with deconvolute was analyzed by software Peak Fit 4.0. Figure 4e showed the structure (surfaces complexes) on boehmite which were (I) (AlO)(PO 2 )-R, (II) (AlO)PO(OH)-R, and (AlO) 2 PO-R, (III) R is C(CH 3 )(OH)PO 3 H 2 , CH 2 N(CH 2 PO 3 H 2 ) 2 for HEDP and NTMP respectively. The non complexed surface was assigned by v P-O and vP-OH at the bands of 960-972 cm À1 and 937-950 cm À1 for both of NTMP and HEDP. Zenobi et al. (2008) reported the ATR-FTIR spectra for HEDP at the identical wavenumbers, while Laiti et al. (1998) investigated the bond distance of O-O in the ions of phosphonate which presented the formation of bidentate complex among the AlOH sites. Li et al. (2021) studied the XPS spectra of La/Zn/Fe 3 O 4 @PAC and illustrated the elements of Fe, Zn, C, La, and O. However, after the HEDP adsorption, in the region of P2 P new binding energy peak was obtained at $133.9 eV and P atomic ratio was increased from 0% to 4.88% suggested the HEDP was eminently adsorbed. The O1s spectra were found at $529.9 eV and $531.  Li et al. (2021) suggested that M-OH contributed to the adsorption of HEDP on the La/Zn/Fe 3 O 4 @PAC surface and gave rise to hydroxyl grouping replacement through HEDP due to ligand exchange. Hill et al. (2019) performed the XPS spectra for desorption of phosphonate, and data presented the 2p phosphorus peaks at 133.7 eV which is a similar finding as reported by Moine et al. (2013) and Bhure et al. (2013) that bindings of phosphonate on the surfaces of metal oxide and alloy). The weak peak of P was estimated after the treatment of NaOH(aq) and chemisorbed phosphonate was removed from the surface during the treatment. The atomic percentage of 0.0 ± 0.0% for P was observed using CASA XPS software which showed that the remaining amount of P was less than the software's detection limit after the treatment of NaOH(aq). This is recommended that from the surface, chemisorbed phosphonate was removed during the treatment. Altaf, Lin, Zhuang, et al. (2021) demonstrated for the first time the detailed XPS mechanism for desorption of NTMP referred to La3d, Zr3d, C1s, Fe2p, and O1s before and after desorption. The XPS mechanism was inner sphere complexation for NTMP desorption, and removal was enhanced by indicating greater binding energy.The NTMP ions contain higher electronegativity, which promotes NTMP desorption via hydroxyl groups. Further, some researchers (Rott, Nouri, et al., 2018;Luo et al., 2015) have recommended the mechanism of phosphonate adsorption, due to the protonation of functional groups electrostatic attraction involved in the adsorption of phosphonates; and simultaneously, in the adsorption process hydrogen bonding interaction would also involve on the adsorbents. Wu et al. (2020) presented the inner sphere complexation mechanism by the ligand exchange, and hydrogen bonding for the sorbents. Thoroughly, in the solution surface hydroxyl groups were replaced by another ligand, and among the metal atom and ligand covalent chemical bond was formed. Also, Hydrogen bonding involves H-bond accepting groups that could be formed the bonds.

Scanning electron microscopy (SEM)
4.4.1. Optimistic influence on phosphonate adsorption Reinhardt, Elordi, et al. (2020) performed an analysis of SEM for GFH adsorbent with the following optimized conditions: The sample was fixed and sputtered by conductive silver and Leica coater, respectively, and then the sample was measured by Gemini microscope. In addition, Ca, Fe, and O were detected by energy dispersive X-ray (EDS) technique (Ultra Dry SDD detector). The result showed that GFH material has a high porosity and crystalline structure comprised by crystals of various sizes which was more important for the adsorption of phosphonates. The Ca origination has an optimistic influence on phosphonates adsorption because the Ca consisting composite was applied. Based on this result, calcium comprising material should be more favorable for phosphpnate adsorption in the future. Altaf, Lin, Zhuang, et al. (2021) investigated SEM analysis with conditions of accelerating voltage-3.00 KV, operational interval-7-3 mm, InLens detector, and magnetic induction-30 KX. The determined SEM analysis presented the spheroidal shape of Zr-La@Fe 3 O 4 , primitive unit cell before desorption of NTMP, and structure was significantly changed as non-primitive unit cell after desorption of NTMP because zirconium formed the agglomerates retained the shape of oxychloride crystal. These findings agree with the similar obtained result of Marote et al. (2002). Moreover, Li et al. (2021) visualized the structure and morphology of La/Zn/Fe 3 O 4 @PAC surface by SEM, and elements characterization was analyzed using EDS. The morphology and elemental distribution of La/Zn/Fe 3 O 4 @PAC contained a porous structure that promoted the loading of hydroxides and metals. This structure could be enhanced the adsorption sites, which might be helpful in the removal of HEDP. The elemental composition existed by Zn, Fe, and La, which implied nanoparticles successfully loaded on the PAC surface.

Brunauer-Emmett-Teller (BET) and Thermogravimetric analysis (TGA)
The BET and TGA descriptions were presented in sections S5 and S6, respectively.

Chemical computation techniques
Model computation techniques can evaluate the bonding mechanism (condensation reactions) between the adsorbate and adsorbent (Hou et al., 2017). Rarely, few researchers has employed chemical computation techniques for phosphonates adsorption mechanism, which is included by (1) Density functional theory (DFT), (2) Molecular dynamic simulation (MDS), (3) Independent Gradient Model (IGM), (4) Hirshfeld surface based Multiwfn. The IGM and Hirshfeld surface based Multiwfn techniques are novel methods to show the weak interaction between phosphonate and adsorbent . Chen et al. (2017b) reported that quantum chemistry simulation using DFT were implemented to estimate the thermodynamic favorability (NTMP adsorption mechanism) of experimental data using DMol3 package of Materials Studio modeling suite. DFT modeling presented that NTMP complex (bidentate binuclear) was DG r ¼ 9.4 to 11.4 kcal/mol, which is greater favorable as energetically. Moreover, the bidentate binuclear complex was more favored via 12 kcal/mol than the monodentate NTMP complex on ferric hydroxide. During the reusability of GFH for NTMP, the energy change is less favorable (DG r ¼ À1.6 kcal/mol is 11.2 kcal/mol) by monodentate complex as shown in equation (13). Hence, high pH eluent will be substantial for desorption of monodentate NTMP complex. Adsorption reaction is shown in equation (14) for bidentate comples (DG r ¼ À19.3 kcal/mol is 5.5 kcal/mol) which is indicated to less energetically favorable. This indicates that the high pH solution should have a lower effect on the desorption of bidentate NTMP complex rather than monodentate NTMP complex. Acelas et al. (2013) selected DFT modeling for adsorption to diminish the computational cost by a threedimensional periodic structure.

Density functional theory (DFT)
5.2. Molecular dynamic simulation (MDS) Zhu et al. (2021) performed the MDS by using the Forcite module via Interface force field (IFF) for PBTC adsorption by minerals (Mt and Kaol), and energy minimization was executed by force field use. To describe the interaction of PBTC and minerals via a microscopic mechanism, density field, mean square displacement (MSD), and Z-density were carried out. It revealed that hydrogen atom and H 2 O molecules have a close and high density on the surface of kaol respectively, contributing to electrostatic interaction. The PBTC density distribution was obtained at 3.0 Å from the Kaol surface, which could be attributed to the H-bonding mechanism. The MSD suggests the higher affinity on Kaol and strong interaction of PBTC on the surface of Mt.

Independent Gradient Model (IGM)
The IGM technique explores the adsorption mechanism to perform the non-covalent interaction between pollutant and adsorbent (Ponce-Vargas et al., 2020). Zhu et al. (2021) recommended that IGM can evaluate the interaction of interfragement (dg inter ) and intrafragement (dg intra ) based on promolecular density. Additionally, they applied interfragement to estimate the interaction between clay minerals and PBTC which obtained the strong attractive interaction (H-bond), and van der Waals (vdw) interaction (weaker interaction than H-bond interaction). Also, weak interaction was found between clay minerals and O atom of carboxyl group through IGM isosurface vividly. Due to H-bond, the strong interaction of Mt and PBTC was observed. Therefore, Mt mineral has concluded superior removal for PBTC.

Hirshfeld surface based Multiwfn
Zhu et al. (2021) implemented this Hirshfeld surface-based Multiwfn technique for the first time to evaluate the weaker interaction between PBTC and clay minerals. They found that the O atom of the carboxyl group indicated the interaction among PBTC and Mt to existing of H-bond character. Furthermore, the fingerprint is an innovative idea to describe the non-covalent interaction between PBTC and clay minerals, demonstrating the strong H-bond donor through the contact of Mt and O atom of the carboxyl group. In conclusion, phosphonate removal and adsorption mechanism via chemical computation techniques should be urgently needed and more exploration is required for the future.

Conclusions and Future perspectives
The increase in phosphonates production, consumption, and contribution of eutrophication has been of great importance in the relevant environment. This review discusses the removal and recovery of phosphonates from wastewater by the low-cost adsorption technique. For practical application, industrial (DTPMP) and municipal wastewater has been treated for phosphonates removal and regeneration. According to data of (1988 to 2018), can be realized how much is needed for the removal, recovery, and reuse of phosphonates as a fertilizer product and industrial production. Even cannot exclude the eutrophication problem. The high removal efficiency was obtained for NTMP with Zr-La@Fe 3 O 4 magnetite, granular ferric hydroxide, ZnFeZr@MP; HEDP (magnetic La/Zn/Fe 3 O 4 @PAC; GFH); DTPMP (GFH); PBTC (montmorillonite) which should be implemented for technical application. Phosphonates contribute as particulate and dissolved phosphorus in the natural water and are also found in WWTP, industrial wastewater, municipal wastewater, and river water. Therefore, in the future should be more focused on the removal and recovery of phosphonates with adsorption, especially for practical practices. For sustainability, desorption/reusability has significantly contributed to adsorbent reuse and adsorbate retrieval. For desorption of phosphonates, an alkaline solution has more applicable for regeneration. Reusability from industrial and municipal wastewater was determined to be successful long term reusability with ZnFeZr. But, lack of systematic desorption study of phosphonates in the literature. Therefore in the future, a systematically desorption study of phosphonates is required. For practical application, the column breakthrough experiment for the NTMP removal from membrane concentrate with granular ferric hydroxide presented the 20% breakthrough has been able to transferability in practical treatment. Also, during the practical treatment, >90% removal efficiency and 78% desorption efficiency of NTMP were achieved by practically column breakthrough treatment from membrane concentrate. So, column breakthrough can be implemented in the future for another phosphonate as a practical application.
The metal ions such as calcium and magnesium have positively affected the adsorption of phosphonates and in the wastewater a higher concentration of calcium and magnesium was found. Therefore phosphonates adsorption should be great attention from the real wastewater with metal ions effect in the future. Generally, most literature presented the lowered phosphonate adsorption efficiency at higher pH. But, the acidic pH was the best for phosphonates removal from wastewater and pH is very important for practical application in agriculture, environmental science, water treatment, and chemical engineering. The characterization technique (FTIR, XRD, TGA, BET, and XPS), and chemical computation techniques (DFT, MDS, IGM and Hirshfeld surface based Multiwfn) played a dominant role in the removal of phosphonates mechanism, which includes the possible bindings for phosphonates adsorption was protonated, deprotonated, monodentate, bidentate; ligand exchange, hydroxyl groups interaction, hydrogen bonding and interfragement mechanism. Some literature lacks a systematic phosphonates adsorption mechanism via adsorbent characterization tools. Hence, characterization techniques and chemical computation techniques should be more focused to explore the adsorption mechanism for the phosphonates removal in the future.

Author Contribution statement
Rubina Altaf contributed with data analysis, paper structure, discussion, paper writing and revision; Bo Sun contributed with paper discussion, structure, paper editing and revision; Huijie Lu contributed with paper discussion, paper revision; Heping Zhao contributed with paper discussion, paper revision; Dezhao Liu contributed with paper conception, paper structure, fund, discussion and revision.