Potential of magnetic quinoa biosorbent composite and HNO3 treated biosorbent for effective sequestration of chromium (VI) from contaminated water

Abstract The present study aims to prepare novel quinoa biosorbent (QB), acid activated QB (QB/Acid) and its nanocomposite with magnetic nanoparticles (QB/MNPs) for batch scale Cr removal from contaminated water. The Cr adsorption was systematically studied at different pH (2–9), adsorbent dosage (1–3 g/L), initial concentration (25–200 mg/L), contact time (180 min) and competing ions in water. Maximum Cr adsorption was observed onto QB/MNPs (57.4 mg/L), followed by QB/Acid (46.35 mg/g) and QB (39.9 mg/g). The Cr removal by QB/MNPs was higher than QB/Acid and QB. Results revealed that the highest Cr removal was obtained at optimum pH 4, 25 mg/L, and 2 g/L dosage. The FTIR spectra displayed various functional groups on adsorbents surface serving as a potential scaffold to remove Cr from contaminated water. The equilibrium and kinetic Cr adsorption data best fitted with Freundlich and pseudo-second order models, respectively (R2 ≥ 0.96). The QB/MNPs showed excellent reusability in five adsorption/desorption cycles (4.7% decline) with minor leaching of Fe (below threshold level). The coexisting ions in groundwater showed an inhibitory effect on Cr sequestration (5%) from water. The comparison of Cr adsorption by QB/MNPs and QB/Acid showed better potential for Cr sequestration than various previously explored adsorbents in the literature. Novelty Statement Quinoa is a cereal crop and after harvesting quinoa straws are either burnt or thrown away which can cause several environmental problems. It would be beneficial to utilize quinoa straws and its modified forms as adsorbents for the water remediation. Therefore, current study aims to estimate the adsorption capacity of quinoa biomass as biosorbent (QB) and its modifications (QB/Acid and QB/MNPs) to treat Cr (VI) contaminated water. The influence of various parameters governing the Cr removal from water has been evaluated. The reusability of QB/MNPs has also been evaluated for its economical use without losing effectiveness for Cr removal from water. The comparison of Cr adsorption by QB/MNPs and QB/Acid showed better adsorption potential for Cr sequestration than various previously explored adsorbents in the literature.


Introduction
The quantity and quality of water reservoirs are adversely affected with industrialization, urbanization and modernization (Tariq et al. 2020;Kumar A et al. 2022). The disposal of industrial effluents into water reservoirs without prior treatment contaminates the surface water and groundwater bodies (Patra et al. 2020;Kumar A et al. 2022). Water contamination with potentially toxic metals (PTMs) has become a global concern due to their bioaccumulation through food chain (Patra et al. 2020;Iqbal MM et al. 2021). Chromium (Cr) is one of the PTMs that enters the environment and poses serious threats to environment and human beings due to its high carcinogenicity, non-degradability, toxicity and mobility (Wang C-C et al. 2016;Zhu et al. 2017). Water contamination with Cr has become a major concern due to its widespread applications in many industrial activities, e.g., metallurgy, metal processing, textile dyeing, electroplating, wood preservation, chrome based manufacturing, leather tanning, and pigment production (Deng et al. 2017;Xie et al. 2017;Khalifa et al. 2019;Suganya et al. 2020). Electroplating utilizes high Cr concentration and releases Cr in effluents up to 2,500 mg/L (Tariq et al. 2020). The permissible Cr level in drinking water set by WHO is 0.05 mg/L (Tariq et al. 2020). The USEPA has recognized Cr as priority hazardous pollutant (Lyu et al. 2017). Chromium is a redox active metal that mainly occurs in two oxidation states, Cr (III) and Cr (VI), the latter being more noxious to human beings and other living things due to its high aqueous solubility and carcinogenicity (Liu W et al. 2014;Din et al. 2021). Thus, there is a dire need to treat Cr-contaminated water. The Cr (VI) reduction looks to be an effective approach for its remediation (Liu F et al. 2017). For Cr (VI) removal, various approaches like precipitation, reverse osmosis, adsorption, chemical reduction, ion exchange and electrodialysis are employed (Wang H et al. 2019;Ou et al. 2020;Tariq et al. 2020;Din et al. 2021). However, due to low efficiency, incomplete removal and high energy consumption these traditional approaches have shortcomings (Gorny et al. 2016;Murtaza et al. 2019;Iqbal MM et al. 2021;Ahmad et al. 2022). Adsorption is considered as an active, flexible, and economically practicable alternative in removing Cr from water (Tariq et al. 2020;Din et al. 2021). Conventional adsorbents such as activated alumina (Mor et al. 2007), zeolite (Basaldella et al. 2007), chitosan polymer (Hena 2010), activated carbon (Gupta et al. 2010), silica gel and bauxite (Shafiq et al. 2018) are expensive to remove Cr from contaminated aqueous systems. However, on large scale these adsorbents are not practicable due to complex production, high cost and associated secondary pollution risks. Most of the current research is focused to evaluate the adsorbent potential of easily available, and low cost adsorbent materials (agricultural residues and lignocellulosic waste) for Cr removal e.g., Eucalyptus camadulensis seeds (Suganya et al. 2020), saw dust, wheat bran (Ogata et al. 2020), rice husk (Shamsollahi and Partovinia 2019), citrus peels (Khalifa et al. 2019), pea pod, tea, ginger and banana peel (Sharma PK et al. 2016), litchi peel (Yi Y et al. 2017), water caltrop shell (Kumar S, Narayanasamy, et al. 2019). However, some adsorbents are difficult to be reused once pollutants are adsorbed on the adsorbent surface from contaminated water. Moreover, to increase the adsorption potential of the adsorbents different surface modifications are practiced (Naushad et al. 2015;Imran et al. 2020;Shah NS et al. 2020;Tariq et al. 2020;Bulgariu et al. 2021). Biosorbents are chemically treated to improve the surface functional groups and integrate the counter ions that improve the adsorption potential of various adsorbents Imran et al. 2020;Suganya et al. 2020). Recently, KOH treated corn straw and Camellia oleifera seed shell (Guo et al. 2018;Ma et al. 2019), phosphoric and acid treated apple peels (Enniya et al. 2018) are examples of few modified adsorbents effectively utilized for Cr (VI) removal from contaminated aqueous systems. Literature shows that the aggregation and suspension of the adsorbents decrease the adsorption potential of the materials (Suganya et al. 2020;Imran et al. 2021). To overcome these limitations as well as to improve the surface characteristics (functional groups, particle size and surface area), composites with metallic nanoparticles are considered as the effective modification to treat contaminated water (Tariq et al. 2020;Iqbal MM et al. 2021;Lucaci et al. 2021). Magnetic composites as adsorbents are widely utilized for the treatment of contaminated water because of its simple preparation and separation after saturation with contaminants (Kumar A et al. 2022). In previous studies, ZnO tetrapod and activated carbon composites were utilized for the effective removal of Cr (VI) and it was found that the Cr removal was 97% at pH 2 (Sharma M et al. 2019). Zhao et al. (2017) utilized magnetic iron oxide/polyacrylonitrile composite for efficient Cr (VI) removal from contaminated water and reported more than 90% Cr removal. Li et al. (2016) found that chitosan magnetic composite effectively removed Cr (VI) from contaminated water at pH 2. Li et al. (2016) revealed that magnetic particles are rapidly separated and easily regenerated by using simple diluted NaOH solutions and reused with minor decline in adsorption capacity.
The current study aims to evaluate the adsorption capacity of quinoa biomass as biosorbent (QB) and its modifications (QB/Acid and QB/MNPs) to treat Cr (VI) contaminated water. Quinoa is a cereal crops and is considered a good replacement for the commercially cultivated grain crops owing to its higher nutritional value (Jarvis et al. 2017;Shabbir et al. 2021). Quinoa is identified as a climate resilient crop against abiotic stress, and it gives higher yield even on marginal lands with low economic input requirements (Ruiz et al. 2016). Quinoa straws after harvesting are either burnt or thrown away which leads to several environmental problems Naeem et al. 2020). Hence, it would be beneficial to utilize quinoa straws and its modified forms as adsorbents for water remediation. The influence of various parameters governing the Cr removal from water has also been evaluated. The equilibrium and kinetic adsorption data have been simulated with equilibrium and kinetic adsorption isotherm models. The impact of coexisting ions in water and reusability of QB/MNPs has also been estimated for Cr sequestration from contaminated water. Moreover, the adsorption potential of QB, QB/Acid and QB/MNPs for Cr removal was compared with the previously used adsorbents in the literature.

Chemicals and reagents
Hydrochloric acid (HCl), nitric acid (HNO 3 ), sodium hydroxide (NaOH), ferric chloride (FeCl 3 ), Ferrous sulfate (FeSO 4 ), potassium dichromate (K 2 Cr 2 O 7 ) and potassium hydroxide (KOH) were used in the batch scale experiments. A 0.1 M NaOH and HCl solutions were utilized to adjust the solution pH during experimentation. A stock solution of Cr (500 mg/L) was prepared with K 2 Cr 2 O 7 salt in distilled water (DW). Further solutions of various Cr concentrations were prepared with dilution method. The chemicals were of analytical grade purchased from Merk, Faisalabad and were used without further purification.

Preparation of QB and its surface modification
For the preparation of quinoa biosorbent (QB), dry biomass of quinoa was collected locally from the field after harvesting. The quinoa biomass was washed with DW thoroughly to get rid of the adhered dirt and unwanted constituents. The air-dried biomass was oven dried for 36 h at 60 C. The oven-dried biomass of quinoa was pulverized to fine powder and preserved for further applications.
To prepare the QB composite with acid (QB/Acid), 25 g QB was immersed in 200 ml HNO 3 solution (5%) with vigorous mixing of the constituents. The mixture was kept on a shaker for 1 h and then filtered using Whatman filter paper. The acid modified QB was washed with DW and again filtered to purify the QB/Acid. The resultant QB/Acid was air dried and kept in the oven at 65 C. While magnetic composite of QB (QB/MNPs) was prepared with precipitation method Iqbal MM et al. 2019;Iqbal MM et al. 2021). A 10% solution of FeSO 4 (w/v) and 10% solution of FeCl 3 (w/v) was prepared in DW. Subsequently, a homogeneous mixture of FeSO 4 and FeCl 3 was prepared by taking 4 mL of FeCl 3 and 8 mL of FeSO 4 in 250 mL beaker and 0.5 g of QB was added and thoroughly mixed. The resultant mixture was titrated against 0.15 M KOH dropwise to prepare the composite QB/MNPs. Subsequently, the solution was filtered, oven dried and pulverized to fine material. The prepared QB, QB/Acid and QB/MNPs were preserved in separate containers for the experiments and characterization.

FTIR analysis and measurement of point of zero charge
It is crucial to determine the functional groups of the prepared QB, QB/Acid and QB/MNPs to understand and justify the adsorption mechanism for the treatment of contaminated systems Shah NS et al. 2020;Imran et al. 2021). The QB, QB/Acid and QB/MNPs used in the current study were characterized with FTIR (Thermo Scientific Nicolet-6700TM, USA) to find the functional groups on their surface. The FTIR spectra of pristine and Cr-loaded QB, QB/Acid and QB/MNPs were achieved in 600-4,000 cm À1 . The QB, QB/Acid and QB/MNPs adsorbents were also characterized by measuring their point of zero charge (PZC) because it is considered as a pH at which the adsorbent is neutral or exhibits zero charge on the surface after the adsorbent interaction with the electrolyte. The PZC of QB, QB/Acid and QB/MNPs adsorbents was estimated by following Imran et al. (2019) and Tariq et al. (2020).

Batch scale adsorption studies
The batch scale adsorption/removal experiments were performed in 250 mL Erlenmeyer flasks at ambient temperature (25 ± 2 C). The effect of different factors i.e., pH (2, 4, 6 and 9), adsorbent dose (1, 2 and 3 g/L), initial concentration (25-200 mg/L) and time (15-180 min) was evaluated on Cr (VI) adsorption onto QB, QB/Acid and QB/MNPs. The Cr containing water was agitated on a mechanical shaker for 3 h at 150 rpm. The residual level of Cr was measured in water samples taken after 15, 30, 60, 120 and 180 min. The Cr removal from contaminated water was optimized for the adsorbent dosage, solution pH and interaction time. The impact of competing ions in water on Cr removal was evaluated by conducting experiment with two groundwater samples (GW 1 and GW2) having characteristics given in Table 1. The GW1 and GW2 were investigated for pH, electrical conductivity (EC), total dissolved solids (TDS), Nitrate (NO 3 À ), sulfate (SO 4 2À ), chloride (Cl À ), Carbonates (CO 3 2À ) and bicarbonates (HCO 3 À ), sodium (Na þ ), calcium (Ca 2þ ) and potassium (K þ ). The concentration of Na þ , K þ and Ca 2þ was determined with Flame photometer. The EC (dS/ m), TDS (mg/L) and pH were measured with a multiparameter meter (3 in one) Hanna meter (HI98129). The concentration (mg/L) of NO 3 À , and SO 4 2À was measured with UV/Vis spectrophotometer (Perkin-Elmer Lambda 25, Germany). The CO 3 2À, HCO 3 À and Cl À concentrations Rð%Þ In Eqs. (1) and (2), C 0 and C f represents the initial and final Cr levels in water after interaction with the adsorbents (mg/ L). In Eq.
(2), q e represents Cr (mg) adsorbed per mass (g) of the adsorbent, M b is the adsorbent mass (g) used for the interaction and V represents the volume of Cr contaminated solution (L). To calculate kinetic Cr adsorption q t at time t onto QB, QB/Acid, QB/MNPs surface, C f in Eq.
(2) was substituted with Cr concentration C t at time t.  (3)) was applied to examine the multilayer adsorption mechanism that occurs on nonuniform surfaces with intermolecular interactions between the adsorbed ions on adjacent active sites Hussain et al. 2020;Sharma G and Naushad 2020).
where K F in Eq. (3) is Freundlich model constant, q e is model adsorption at equilibrium and 1 n indicates adsorbent heterogeneity on its surface. Linear form of Freundlich model (Eq. (4)) was employed to determine the model parameters from the intercept ðK F Þ and slope 1 n À Á of the respective curve between ln(q e ) versus ln (C e ) (Vigneshwaran et al. 2021).
Langmuir model (Eq. (5)) was applied to examine the monolayer Cr adsorption on finite number of adsorption sites onto QB, QB/Acid and QB/MNPs surface (Vilela et al. 2019;Din et al. 2021).
In Eq. (5), q e is adsorption capacity of adsorbents (QB, QB/Acid and QB/MNPs) at equilibrium, C e is equilibrium concentration, K L is Langmuir constant and q max is the maximum Cr adsorption at equilibrium. The linear form of Langmuir model (Eq. (6)) was used to estimate Langmuir model parameters (Ahmad et al. 2017;Imran et al. 2020) from the slope 1 q max and intercept 1 K L q max of the respective curve for Cr adsorption on QB, QB/Acid and QB/MNPs.
In Temkin model (Eq. (7), b (J/g/mol) and K T (kJ/mol) indicate equilibrium-binding-constants in Temkin model, T (K) is the temperature and R is universal gas constant. The values of Temkin model parameters (b and K T ) were estimated from the slope RT b À Á and intercept RT b À Á K T of the curve between q e and ln ðC e Þ: Moreover, according to linear form of DR model (Eq. (8)), a graph between lnðq e Þ versus e 2 was plotted to calculate the values of DR model parameters (q m , k DR ) from the respective slope and intercept of the curves for Cr adsorption onto QB, QB/ Acid and QB/MNPs.
The kinetic experimental Cr (VI) adsorption onto QB, QB/Acid and QB/MNPs (at 50 mg Cr/L) was simulated with pseudo 1 st order (Eq. (9)), pseudo 2 nd order (Eq. (10)) and intra-particle diffusion (Eq. (11)) models. logðq e À q t Þ ¼ log ðq e Þ À k 1 2:303 Where q t is Cr adsorption at time (t), q e is Cr adsorption at equilibrium and k 1 is pseudo 1st order rate constant. A graph was drawn between t versus logðq e À q t Þ, to find the values of first order model parameters from the intercept ð log ðq e ÞÞ and slope k 1 2:303 of the respective curve. In Eq. (10), k 2 is pseudo 2nd order rate constant. A graph was drawn between t versus t q e to estimate the pseudo 2nd order model parameters from the intercept 1 k 2 ðq e Þ 2 and slope 1 q e of the curve. Moreover, in Eq. (11), k d is the diffusion model constant and C is the boundary layer thickness . The values of intra-particle diffusion model parameters were estimated from the slope and intercept of the curve for Cr adsorption onto QB, QB/Acid and QB/MNPs.

Reusability and stability of QB/MNPs
It is crucial to determine the adsorbents reusability and stability for removing certain contaminant from the contaminated water. Therefore, based on the maximum Cr adsorption potential of QB/MNPs, only QB/MNPs was tested for the reusability in five adsorption and desorption cycles at 50 mg Cr/L, pH-4, dose 2 g/L and equilibrium. The QB/MNPs with attached Cr were recovered by rinsing it with HCl (4%) solution followed with DW washing after each adsorption cycle. The rinsed QB/MNPs were filtered and oven-dried to reuse it for Cr sequestration from contaminated water. The residual Cr concentration after each adsorption cycle was measured to evaluate its reusability potential and was compared with the first cycle. The QB/ MNPs stability was evaluated by measuring the Fe concentration leached during acid washing of Cr containing QB/ MNPs (Hussain et al. 2020;Iqbal MM et al. 2021).

Ftir analysis and point of zero charge measurement
The FTIR analysis was carried out to find the functional groups involved in Cr (VI) removal onto the surface of QB and its modified forms (QB/Acid, QB/MNPs) (Figure 1). The spectral range for FTIR analysis was taken from 600-4,000 cm À1 for the Cr (VI) removal by QB, QB/Acid, and QB/MNPs. The broad peaks at 3,328 cm À1 matches with O-H alcoholic functional group and 2,909 cm À1 indicates alkane groups (C-H) . The transmission at 1,704, 1,689 and 1,696 cm À1 corresponds to weak stretching due to alkenes (C ¼ C) (Patra et al. 2020). Clear transmission bands were visible at 1,010-1,104, 1,175-1,190, 2,909 and 3,328 cm À1 . The FTIR spectra showed that QB/MNPs did not possess OH and C-H groups. Absorbance at 1,010-1,190 cm À1 indicates C-O and C-N Imran et al. 2021). The FTIR spectra revealed a change in the transmission bands depending on the Cr loading and nature of the adsorbent. The Cr-loaded adsorbents demonstrated more absorbance on the adsorbent surface. In the functional group region, all the adsorbents showed peaks almost at the same wavenumber, but the absorbance varies for Cr-loaded and unloaded materials. The absorbance was maximum for QB/MNPs as compared with QB/Acid and QB either in the presence or absence of Cr.
The PZC values for QB, QB/MNPs and QB/Acid were 7.1, 6.1 and 5.4, respectively. Literature shows that for anionic pollutants (e.g., Cr as chromate), contaminant adsorption is high if aqueous solution has pH < PZC. It is possibly due to the protonation of the adsorbent surface (H þ ) at low pH (

Effect of initial Cr concentration
The initial Cr concentration plays a key role for its sequestration by an adsorbent at a fixed dose. Figure 2 demonstrated the effect of initial Cr concentration (25-200 mg/L) on the removal and adsorption of Cr by QB, QB/Acid, and QB/MNPs. The results depicted that the Cr removal decreased by 69-40%, 90-46%, and 97-57% correspondingly for QB, QB/Acid, and QB/MNPs when Cr concnetration increased from 25 to 200 mg/L. The highest Cr removal (97%) was attained by QB/MNPs at lower concentration (25 mg Cr/L). High Cr concentration linearly increased adsorption capacities of the adsorbents while decreased Cr removal from contaminated aquesous system. There is a quick increase in Cr adsorption at initial stage because of great interaction of Cr molecules with the adsorption sites and electrostatic attraction which resulted into gradual increase in Cr adsorption at higher concentration. The Cr adsorbed by QB, QB/Acid and QB/MNPs was 39.9, 46.35 and 57.4 mg/g, respectively. Higher Cr concentration acts as a governing force to reduce the resistance of mass transfer of Cr molecules among solid and liquid phase (  concentration favors the formation of oxide/hydroxide (Aigbe and Osibote 2020) and improves Cr adsorption at its elevated concentrations. Figure 3a presents the impact of contact time (15-180 min) on Cr removal by QB, QB/Acid, and QB/MNPs at constant adsorbents dose (2 g/L), initial concentration (50 mg Cr/L) and pH-4. The results disclosed that Cr removal increased by 41-52%, 52-69%, and 68-83% respectively by QB, QB/ Acid, and QB/MNPs when the contact time increased from 15-180 min. The maximum removal was obtained in first 30 mins and then a minor increase was observed with increasing contact time which became steady to attain equilibrium (Figure 3a). It might be credited to high surface area and availability of more adsorption places during first 30 min that can readily uptake Cr ions (Tariq et al. 2020). With time, the repulsive forces became dominant between the solid and liquid phase and there is decline in the removal rate (Pradhan et al. 2019;Din et al. 2021). Overall, results revealed that maximum Cr removal with QB, QB/Acid and QB/MNPs adsorbents used in the current study was achieved in first 30 min of interaction between the adsorbents and Cr in simulated contaminated water.

Effect of adsorbent dose
The optimization of material dosage in adsorption experiment is a central parameter that controls the capacity of an adsorbent for the efficient removal of contaminants from aqueous systems Iqbal MM et al. 2021). Figure 3b represents the effect of QB, QB/Acid and QB/MNPs dosage (1-3 g/L) on Cr removal at 50 mg Cr/L, pH-4 and equilibrium. The results revealed an increasing trend in Cr removal with an increase in QB, QB/Acid and QB/MNPs dosage. The increase in Cr removal was: 39-60%, 55-78%, and 71-85%, respectively for QB, QB/Acid, and QB/MNPs when dose was varied from 1-3 g/L. This increase in Cr removal with dose is attributed to the availability of the more active sites for Cr adsorption at higher dose (Shakya and Agarwal 2019;Hussain et al. 2020). At lower adsorbent dosage, there exists a competition between Cr ions to occupy the available sites (Tariq et al. 2020). However, at higher adsorbent dosage, there are plenty of active sites available for the Cr and competition decreases leading to higher removal efficiency at elevated dose of each adsorbent (Aljerf 2018). The difference in Cr removal (%) was higher when the adsorbent dosage was changed from 1-2 g/L as compared with Cr removal when adsorbent dosage changed from 2-3 g/L due to occupation of more active sites and decline in active surface area of sorbent per unit mass. Therefore, the optimum dose considered for the Cr adsorption onto QB, QB/Acid and QB/MNPs was 2.0 g/L. The highest Cr removal of 85% was observed by QB/ MNPs at 2 g/L. The increase in Cr removal by QB/MNPs is attributed to the improvement in its total surface area, surface characteristics and the availability of active exchangeable sites for the attachment of Cr ions. There is decline in Cr adsorption at higher dose (data not presented) due to limited availability of Cr ions to occupy the adsorption sites at higher dosage of the adsorbent (Patra et al. 2020).

Effect of solution pH
The pH of aqueous Cr (VI) solution plays a vital role in its sequestration by the adsorbents due to surface protonation and deprotonation. Figure 4a shows the influence of solution pH (2-9) on Cr removal by QB, QB/Acid, and QB/MNPs keeping other parameters constant i.e., Cr concentration (50 mg/L), time (180 mins) and dose (2 g/L). Results show that the Cr removal increased when pH was raised from 2 to 4. However, Cr removal diminished by 83-67%, 69-39% and 52-35% with QB/MNPs, QB/Acid, and QB, respectively, when pH varied from 4 to 9 which confirmed a clear dependence of Cr removal on the system pH. Chromium removal was maximum at pH 4 because the surface of adsorbents was positively charged at this pH and HCrO 4 À ions showed more electrostatic pull toward adsorbents surface due to protonation (Aigbe and Osibote 2020; Tariq et al. 2020. In acidic media, the adsorbent surface is highly protonated (excess H þ presence) (Huang et al. 2020). Hence, great electrostatic attraction occurs between the positively charged surface and anionic Cr as CrO 4 2À , thereby facilitating the adsorption at low pH Aigbe and Osibote 2020). Contrarily, higher pH with more OH ions showed reduction in Cr adsorption and its removal (Ali A et al. 2016;Tariq et al. 2020) due to deprotonation of the adsorbent surface. Noticeably, highest Cr removal (83%) was achieved by QB/MNPs as compared to QB/Acid, and QB due to dual properties of biosorbent and MNPs. This high Cr adsorption onto QB, QB/Acid and QB/MNPs is also justified from PZC because adsorption of an anionic contaminant is favored by the adsorbents at pH < PZC Iqbal MM et al. 2021).

Impact of coexisting ions in water
The natural groundwater contains various cations and anions, and their coexistence greatly affects the adsorption of aqueous Cr (chromate) onto the adsorbent surface (Maitlo et al. 2019;Zeng et al. 2021). In the current study, the impact of coexisting anions (NO 3 À , SO 4 2À , Cl À , CO 3 2À and HCO 3 À ) and cations (Na þ , K þ , Ca 2þ ) on Cr removal by QB, QB/Acid and QB/MNPs was evaluated by using two groundwater samples (GW1 and GW2). The concentration of different cations and anions in GW1 and GW2 is given in Table 1. Figure 4b displays the impact of coexisting ions on Cr removal at optimum pH 4, dose 2 g/L and 50 mg/L initial Cr concentration under equilibrium. The inherent Cr concentration in GW1 and GW2 was 0.011 and 0.025 mg/L, respectively. The maximum decline in Cr removal with QB in GW1 and GW2 was 4.66% and 2.16% and while with QB/Acid respective decline in Cr removal was 5.96% and 2.7% as compared with synthetic water (SW). But decline in Cr removal with QB/MNPs in GW1 and GW2 was 4.9 and 2.67% as compared with SW. The results (Figure 4b) revealed a more decline in Cr removal by GW1 onto QB, QB/Acid and QB/MNPs as compared with GW2 which is accredited to higher concentration of anions in GW1 than GW2. Maitlo et al. (2018) evaluated the impact of different concentrations of interfering ions on the efficiency of adsorbents for the treatment of Cr contaminated water. They found that the presence of PO 4 in water significantly hindered the Cr adsorption onto iron hydroxides as compared with silicate and other ions present in natural water. The results of the present study are in line with the previous finding that the presence of competing anions in water inhibited the Cr (VI) removal Zeng et al. 2021).

Reusability and stability of QB/MNPs
The stability and reusability potential of QB/MNPs for the Cr adsorption was evaluated due to maximum Cr adsorption onto QB/MNPs as compared with QB and QB/Acid. Figure 4c presents the results for Cr removal when QB/ MNPs was evaluated for its reusability in five adsorption/ desorption cycles at Cr concentration of 50 mg/L under optimum dose and pH. The results revealed a 4.7% decline in Cr removal after five adsorption/desorption cycles which indicates cost effective reuse of QB/MNPs composite for Cr sequestration from aqueous system. There was 1.0, 1.82, 2.32, 3.52 and 4.7% decline in Cr removal in 1st, 2nd, 3rd, 4th and 5th desorption cycle. Likewise, Fe concentration leached from QB/MNPs during its recovery in 1st, 2nd, 3rd, 4th and 5th desorption cycle was 0.05, 0.07, 0.08, 0.11 and 0.13 mg/L which are lower than the permissible revel of Fe  (Langmuir, Freundlich, Temkin and Dubinin-Radushkevich) and kinetic adsorption (Pseudo 1st order, Pseudo 2nd order, and intra-particle diffusion) models. in water as per WHO limits. It is concluded from the reusability and stability results that QB/MNPs can be well reused upto certain number of adsorption/desorption cycles for the Cr removal from water without losing its effectiveness.

Modeling of equilibrium and kinetic adsorption
Freundlich, Langmuir, Temkin and DR isotherm models were employed for the simulation of equilibrium adsorption experiments. The value of regression coefficient (R 2 ) was chosen to determine the suitability of model. The correlation behavior of equilibrium adsorption models for Cr adsorption onto QB, QB/Acid and QB/MNPs has been presented in SS Figure 1. The values of model parameters including R 2 have been provided in To evaluate the kinetics of Cr adsorption onto QB, QB/ Acid and QB/MNPs, pseudo-first order, pseudo-second order and intra-particle diffusion models were employed. The values of kinetic models' parameters including R 2 are presented in Table 2 while models fitting behavior with experimental data have been shown in SS Figure 2. The simulation results for Cr adsorption indicated decreasing trend in k 1 onto QB, QB/Acid and QB/MNPs. Similarly, the values of k 2 were 0.25, 0.1 and 0.11, respectively for QB, QB/Acid and QB/MNPs. The maximum k d value (0.39) of intra-particle diffusion model was obtained with QB/Acid. The comparison of kinetic models' response for Cr adsorption based on the value of R 2 revealed that the kinetic experimental adsorption data with all the three adsorbents (QB, QB/Acid and QB/MNPs) was well explained with pseudo-second order model (R 2 ¼ 0.99). The trend of models' suitability for kinetic adsorption of Cr was pseudo-second order > pseudo-first order > intra-particle diffusion. Similar response for kinetic adsorption of Cr with pseudo-second order was obtained by Tariq et al. (2020) and Din et al. (2021).

Comparison of adsorption potential with literature
The Cr adsorption potential onto QB, QB/Acid and QB/ MNPs nanocomposite from contaminated water has been compared with various other previously explored adsorbents ( Table 3). The comparison with literature shows that QB modification with acid (QB/Acid) and its magnetic nanocomposite (QB/MNPs) exhibited relatively more potential for Cr adsorption than several other adsorbents e.g., Calcite based biocomposite (Mishra et al. 2020), Algal bloom/Ironcarbon composites (Cui et al. 2019), Chitosan carbonized rice husk (Sugashini and Begum 2013), food materials/Citric acid , trimethyloctadecylammonium bromide impregnated on Artemisia monosperma (Ali HM et al. 2021) and activated carbon/micro-sized goethite composite . The acid treated lantana camara (Nithya et al. 2020) showed higher Cr adsorption (362.8 mg/ g) as compared with nanocomposite QB/MNPs used in the present study (Table 3).

Conclusions
The quinoa biosorbent (QB), its composite with magnetite nanoparticles (QB/MNPs) and acid activated QB (QB/Acid) were prepared and successfully applied for Cr (VI) removal from contaminated water. The FTIR analysis of pristine and Cr loaded adsorbents showed effective Cr loading onto the adsorbents surface. The Cr removal was greatly varied with the type of biosorbent, solution pH, Cr concentration, competing anions and contact time of adsorbent-adsorbate interaction. The optimum pH and dosage of the adsorbents (QB, QB/Acid and QB/MNPs) for highest Cr sequestration from water were pH 4, and 2 g/L, respectively. The maximum Cr removal was obtained in first 30 min due to saturation of the activated sites. The QB/MNPs showed highest Cr adsorption of 57.4 mg/L followed by QB/Acid (46.35 mg/g) and QB (39.9 mg/g). The presence of coexisting ions in water revealed a decline in the Cr removal from contaminated water. The experimental equilibrium isotherm data fitted well with Freundlich isotherm model while kinetic Table 3. Comparison of adsorption/removal potential of the adsorbents used in the present study with the previously explored adsorbents in the literature.

Materials
Adsorption ( isotherm data was well explained by pseudo-second order model. The reusability of QB/MNPs showed 5% reduction in Cr removal after five adsorption/desorption cycles. The obtained results proved that QB/MNPs composite is an inexpensive adsorbent with a higher reusability and adsorption potential for the sequestration of Cr from contaminated water.
Disclosure statement