Sulphate radical-based advanced oxidation technologies for removal of COD and ammonia from hazardous landfill leachate: A review

ABSTRACT Solid waste landfill (SWL) is known as a major pollution source contaminating air, water sources, and soil environment. The main hazard of SWL is leachate including high organic matter and nutrients. Recently, sulphate radical-based advanced oxidation processes (SR-AOPs) based on persulphate (PS) have been applied as a useful method for removal of chemical oxygen demand (COD) and ammonia from landfill leachate (LL). The removal efficiency of the SR-AOPs depends on effective parameters such as pH, oxidant dosage, catalyst dosage, temperature, reaction time, and so forth. Among SR-AOPs, thermal persulphate oxidation (TPO) seems to be promising for removal of both COD and ammonia; however, the high amount of energy cost is regarded as a major drawback. The drawbacks and future challenges for the use of SR-AOPs for LL have been discussed, and some strategies such as using coupled mechanisms of SR-AOPs have been proposed to tackle the common difficulties. Finally, the performance of SR-AOPs for treatment of LL has been compared with hydroxyl radical processes.


Introduction
Population growth, industrial revolution, and an increase in living standards have been accompanied by an explosion of municipal and industrial waste.Over years, a wide range of strategies have been introduced to relieve the adverse effects of municipal and industrial waste.Landfilling is regarded as a dominant alternative for the disposal of solid waste among disposal methods such as dumping, incineration, gasification and so forth [1].
Given the potential risks of LL to the environment, especially groundwater, environmental regulatory agencies are forced to apply more stringent standards for discharging leachate into water bodies.There are lots of treatment processes, biological processes (aerobic, anoxic and anaerobic), and physical-chemical processes such as coagulation/ flocculation (C-F), adsorption, chemical oxidation, sedimentation/flotation, chemical precipitation in order to reduce the environmental impacts of the leachate [6].Biological processes are commonly preferred because of the presence of a wide range of biodegradable substances in LL due to the significant BOD 5 /COD ratio, especially in immature leachate which has high biodegradability ratio.However, biological treatment is rarely able to fulfill the discharge standards since there are high concentrations of organic matters and refractory substances (mostly humic and fulvic acids) in LL [7].Recent years, advanced oxidation processes (AOPs) such as Fenton oxidation, electrochemical oxidation, ozone oxidation, ultraviolet (UV)/hydrogen peroxide (H 2 O 2 ) and photo Fenton which are based on hydroxyl radicals ( • OH) have been widely utilised for treatment of LL [8,9].However, hydroxyl radicals are unstable and have a short life span of fewer than 10 −3 seconds [10].In addition, H 2 O 2 which is commonly used as oxidant in the conventional AOPs, such as Fenton, has a lower stability because it tends to rapidly engage in the compounds present in the environment instead of reacting with catalysts to produce strong free radicals which are able to remove the main pollutants from the soil or water resources [11].Recent  , have been successfully used for the treatment ofLL [12].Many studies have reported that AOPs based on sulphate radicals have a great ability to eliminate the different contaminants from pharmaceutical [13], pulp and paper [14], slaughterhouse wastewater and in particular LL [15].The sulfate radicals have proved powerful oxidants for the removal of ammonia and organic matters from LL, whereas the effectiveness of other hydroxyl radical based-AOPs (HR-AOPs) on the removal of ammonia from LL is not significant [15].The materials presented in this study will cover the following objectives related to SR-AOPs: (1) the study of ammonia and COD removal from LLthrough sulphate radicals, (2) the assessment of removal efficiency of the various SR-AOPs for LL treatment, (3) the evaluation of effects of main parameters on each SR-AOPs (4) the comparison of the performance of SR-AOPs with HR-AOPs, and finally (5) the challenges and drawbacks of SR-AOPs.

LL composition
TheLL may be defined as a water-based solution having four types of contaminants: the dissolved organic matters (DOMs), inorganic macro-components, heavy metals, and anthropogenic-specific organic compounds.DOMs are considered as a chain of organic molecules with various molecular weights, i.e. low molecular weight substrates such as amino acids, carbohydrates, organic acids, and also some high molecular weight substrates such as humic substrates [9,16].In addition, other major contaminants include inorganic components consisting of the ions of ammonium (NH 4 + ), iron (Fe 2+ ), calcium (Ca 2+ ), sodium (Na + ), magnesium (Mg 2+ ), potassium (K + ), manganese (Mn 2+ ), chloride (Cl − ), sulphate (SO 4 2− ) and bicarbonate (HCO 3 − ) with heavy metals like cadmium, chromium, arsenic, cobalt, copper, nickel, lead, mercury, and zinc.In addition, the xenobiotic organic compounds may be produced by the household and industrial chemicals and sludge of treatment plants, having a high amount of aromatic hydrocarbons, phenols, and chlorinated aliphatic compounds [17,18].

Persulphate salts
There are two main sulphate sources for SR-AOPs, including peroxydisulfate (PDS), which is commonly known as the persulphate (PS), and peroxymonosulfate (PMS) [19].PS mainly exists in three main substances including sodium, potassium, and ammonia persulfates.Among them sodium persulphate is more stable and cost-effective [20].The studies have shown that the sulphate radicals using Ps are viable process of removing refractory dissolved organic matters and ammonia from leachate [15,21].As a best of authors' knowledge, according to Figure S1, sodium persulfate was applied in most studies conducted about the treatment of LL, whereas potassium persulfate has been utilised in fewer studies.In addition, ammonia persulphate is not favourable due to the increase of ammonia concentration in the solution [19].

The comparison between non-activated and activated PS for LL treatment
It has been indicated that PS alone does not have sufficient oxidation power compared to its derivativesin particular, sulphate radicals having high oxidation potential [22].In order to generate sulphate radicals, it is necessary to inducePS through various activation methods [23][24][25][26][27]. Amr et al. [28] pointed out that the COD and ammonia removal efficiencies by using only PS were less than the combination of PSwith Al 2 (SO 4 ) 3 for the treatment of LL.Accordingly, the removal efficiency of COD, ammonia, and colour by using PS were about 33%, 45%, and 19%,whereas the removal efficiencies of activated PS increased to around 68%, 81%, and 48%, respectively.There are common activation processes for PS applied for treatment of LLincluding thermal persulphate oxidation (TPO), metal persulphate oxidation (MPO), electrochemical persulphate oxidation (EPO), activated carbon persulphate oxidation (APO), hydrogen peroxide persulphate oxidation (HPO), ozone persulphate oxidation (OPO), and radiation persulphate oxidation (RPO) falling into three categories including ultraviolet (UVPO), microwave (MRPO), and ultrasound (URPO).

COD and ammonia removal mechanism by SR-AOPs
The removal mechanisms of organic matters using sulphate radicals are based on hydrogen abstraction, double bond, and electron transfer [29].According to these removal mechanisms, PS is consumed in two ways.Firstly, organic or inorganic materials(R) may be directly oxidised by PS (Equation 1) or they may react with sulphate radicals which can convert pollutants into less harmful materials (Equation 2) [29,30].
According to Equations 3-9, ammonia is degraded as a result of the reaction with the sulphate radicals and then converted into nitrate ions [31].
According to Equation (3), at first, ammonia reacts with sulphate radicals and transforms into NH N 2 and HSO À 4 followed by some sequence reactions in which nitrite ions turn into nitrate ions after reacting with sulphate radicals along with other oxidants such as O 2 , O* (activated oxygen), and OH during oxidation process (Figure S2) [31].
Furthermore, the removal of ammonia by sulphate radicals is attributed to the presence of Cl − ions in LL, which may accelerate the degradation of ammonia [32].The concentration of chloride ions in LL naturally varies from 200 to 3000 mg/L for the young LL to 100-400 mg/L for the old ones [33].Accordingly, Cl, which is produced as a result of the reaction of sulfate radicals with Cl Equation 10 (reacts with Cl − ions) Equation 11(resulting in Cl À 2 , which is less reactive than sulphate radicals.In addition, after the generation of Cl 2 based on Equations 12 and 13, hypochlorous acid (HOCl), which is formed due to the presence of H + ions in LL, may cause oxidising ammonia (Equations 14-16) [21].

Thermal persulphate oxidation (TPO)
TPO is a combination of heat and PS injection into an aqueous environment by which the sulphate radicals will be produced based on Equation 17 [34].
The direct effects of thermal activation are related to stripping ammonia or other volatile compounds and breaking the bonds of contaminants by heating, while the indirect effects of heating is associated with the production of free radicals [15].Chen et al. [21] revealed that the ammonia and COD removal from amature L Lby sulphate radicalsby TPO reached 99% at the operating temperature of 60°C.In another study, Deng and Ezyske [15] reported that the removal efficiency of COD (COD 0 : 1254 mg/L) and ammonia (initial ammonia: 2000 mg/L) by TPO were obtained 100% and 91%, respectively.In contrast to Chen et al. [21], Deng and Ezyske [15] indicated that the removal efficiency of COD and ammonia decreased by increasing pH from 3 to 8.3.It may be attributed to the fact that by increasing pH from acidic to alkaline condition, the amount of sulphate radicals is reduced due to the generation of radical scavengers such as HCO : 3 and CO À : 3 during oxidation process (Equations 18 and 19) [35].
On the other side, it may also be speculated that the removal efficiency of COD and ammonia in alkaline condition is more than acidic condition because of the generation of more hydroxyl radicals simultaneously along with sulfate radicals in alkalinity condition, as by increasing the pH to neutral or alkaline pH, the sulphate radicals react with H 2 O and OH − , resulting in more concentration of hydroxyl radicals (Equations 20 and 21) [36].
The alkaline pH is generally considered as the optimal pH for ammonia removal [40], as the distribution coefficient of NH 3 α NH3 ð Þ, which represents fraction of NH 3 , is more than NH 4 + ions at pH above 7, so there would be more ammonia in wastewater which can react with free radicals [41].
Another main parameter is the reaction temperature, which has an essential role in the decomposition of pollutants by heat-activated PS [42].The fission of the O-O bonds of PSdepends on the operating temperature in TPO, and the rate of PS decomposition will rise by increasing temperature [43].Moreover, when the temperature of wastewater increases to boiling point, gas bubbles will be produced, emitting ammonia into the atmosphere [44].In addition, it is speculated that by increasing temperature, the liquid mass transfer coefficient (k L ) and the gas transfer coefficient (k G ) will increase, leading to more ammonia removal efficiency [45].
Despite the fact that the high temperatures are beneficial to more degradation of organic compounds by TPO, the radical scavenging phenomenon may emerge, which acts as the sulphate radical consumer, and then the efficiency of thermal activation may be reduced [39].In order to run TPO in high operating temperature, a considerable heating energy is required, leading to high operating costs.The optimum temperature for the treatment of LL by TPO has been commonly reported between 50°C and 60°C [15,21].
Althoughthe increase of PS concentration leads to the further PS radical production sulfate radicals may act as radical scavengers at high concentration levels (Equation 26) and also react with PS and generate sulphate anions (Equation 27), leading to the reduction in the number of sulphate radicals and then removal efficiency [46].Generally, TPO is regarded as rapid process and 1-2 hours have been reported as an appropriate reaction time for TPO [39,47].
Among the various transition metals, Fe o (zero-valent iron (ZVI)), Fe 2+ , and Fe 3+ arewidely considered as the homogeneous, non-toxic, and more cost-effective activators [56].Some forms of Fe ions, such as ferrate(VI) can directlyoxidize ammoniato N 2 or NO 3 − Other forms of iron ions can indirectly oxidise contaminants by activation of PS.In one study, Kattel and Dulova [57] carried out a study about the PS activation by ferrous ion, and the removal efficiency of COD and (dissolved nitrogen) DN were about 36% and 39%, respectively.One of the major disadvantages of activation PS with transition metals is the accumulation of M (n+1) in the solution, resulting from the slow regeneration of M (n+1) to M n+ based on Equation 29.In MPO, the slow transformation of Fe 3+ into Fe 2+ is likely to occur, through which the ferric sludge may be generated [58].Hence, some studies suggested that adding the agents such as hydroxylamine or using complement processes such as UV or electricity along withMPO can enhance the regeneration rate of catalyst [58,59].Zhang et al. [60] applied Fe 2+ /PDS in combination with the electrochemical process for LL to reduce Fe 3+ to Fe 2+ on the cathode surface quickly, increasing the degradation rate.Kattel and Dulova [57] that adding sodium thiosulphate (Na 2 S 2 O 3 .5H 2 O) accelerated the conversion of ferric ions into ferrous ions according to Equations 30, increasing the removal efficiency of COD and DN up to 15% and 5%, respectively.ZVI has a great potential for removal of ammonia and organic matters [31].PS can be activated on the surface of ZVI when ZVI transforms into dissolved Fe 2+ ions due to the reaction with dissolved oxygen [61].In addition, ZVI can reduce nitrate to nitrite and then ammonium and nitrogen gas, since the nitrate and nitrite can obtain the electrons on the surface of ZVI [62].
Generally, acidic pH, ranging from 3 to 5, is considered as favourable pH for MPO because the hydroxy complexes of metal catalysts are likely to form at extreme alkaline pH [63].Indeed, the number of sulphate radicals may decrease as a result of the production of oxyhydroxides of Fe 3+ such as FeOH 2+ , Fe(OH) 2+ , and Fe 2 (OH) 24+ [64].Recently, Antony et al. [65] assessed the potential of zero valent aluminium (ZVAl) in the combination with PS and H 2 O 2 for the treatment of stabilised LL, by which the COD removal reached 83% at washing time of 20 min, ZVAl dose of 10 g L −1 , and acidic pH of 1.5.
In some studies, metal transitions like Fe or Co were coated on the catalyst bed.Guo et al. [66] introduced Fe 2 O 3 /Co 3 O 4 /EG (Exfoliated Graphite) as a synthetic catalyst for the activation of PS which was able to be recovered due to its magnetic properties, and the results showed that 67.1% COD and 90.6% ammonia removal from LL were obtained.But the removal efficiency of ammonia and COD changed slowly after optimised concentration of PS, which may be attributed to the emergence of scavenging phenomenon, resulting in the consumption of SO 4 •-and PS, according to the Equations 26 and 27 [64,67].
Recently, some cost-effective sources have been introduced to tackle the challenges in using metal catalysts.In order to reduce the production of chemical waste in MPO process, Liu et al. [68] introduced a new magnetic composite coagulant of MFPFS formed from the combination of Fe 3 O 4 nanoparticles and polymeric ferric sulphate (PFS) followed by MPO by which the removal efficiency of COD, BOD, ammonia and total nitrogen at the optimum point reached 74%, 24%, 18%, and 35%, respectively.Soubh et al. [69] applied converter sludge of a steel company which contained iron oxide compounds such as Fe 3 O 4 and Fe 2 O 3 to activate PS for the treatment of synthetic and real LL, and the removal efficiency of COD and ammonia for the synthetic LL were 73.56% and 63.87% and for the real LL were 48.38%, and 37.98%, respectively.

Electrochemical PS oxidation (EPO)
EPO oxidises pollutants through two ways, including direct and indirect oxidation.In direct oxidation, the electron of pollutants is transferred to the surface of the anode electrode, whereas in indirect oxidation, pollutants are oxidised by hydroxyl and sulphate radicals [70].According to Equation 31, PS can be electrochemically activated on the cathode surface to generate sulphate radicals [71,72].
Song et al. [73] reported that the concentration of nitrate and ammonia after electrochemical process were 81% and 100%, respectively.In fact the nitrate as an electron donor is transformed into nitrite, ammonia, and eventually nitrogen gas [73].
Zhang et al. [60] reported thatEC/Fe 2+ /PDS process with 67.7% COD removal efficiency (COD o : 1900 mg/L) had higher performance than ECalone (COD removal: 28.1%) and Fe 2 + /PDS (COD removal: 44.8%).They also indicated that more than 81% of PS was sufficiently activated at the presence of electricity, while this was just 23% of PS without using electricity during 240 minutes of oxidation reaction.This is because that PS is regenerated through Equation 32 on anode where sulphate ions are converted into PS [74].
LL has naturally high electrical conductivity because of having a wide range of anions and cations.However, some substances, such as chlorine compounds including chlorine perchloric acid (HClO 4 ) and NaCl along with other oxidants are added to wastewater to increase the concentration of Cl − ions [75,76].Pe´rez et al. [77] showed that high concentration of chloride ranging 5000 to 20,000 mg/L, resulted in higher concentration of chlorate and perchlorate, leading to the higher ammonium removal efficiency in comparison to low concentrations of chloride (<2000 mg/L).At the presence of Cl − ions, the Cl 2 is produced on anode, leading to the generation of hypochlorous acid (HOCl) and hypochlorite ion (ClO − ),which have a major role in degradation of ammonia and organic matters [78].
Based on Equations 33 and 34, HOCl and ClO − can oxidise the ammonium into N 2 , especially in acidic condition [79].
Acidic pH, from 3 to 6, is introduced as a suitable range of pH for EPO [65,74,75,80,81].The direct effect of pH on the EPO is related to the over-potential of oxygen evolution reaction (OER) in which electrolysis of water occurs at 1.2 V vs. NHE (Normal Hydrogen Electrode) [78].At acidic pH, the over-potential of the system is enhanced by the increase in the H + ions in the solution, causing more degradation rate of organic matters through directly anode oxidation [82].Varank et al. [83] reported that the COD removal reached about 72.6% by EPO for the treatment of leachate nano filtration concentrate at pH of 5, S 2 O 8 2-/COD ratio 1.72, current 1.26 A, and reaction time 34.8 min.The current density has high influence on EPO, as it directly relates to the radical activation and direct oxidation.In general, the applied current density varies based on the concentration of pollutants and the amount of EC in the wastewater.High current densities accelerate the activation of PS and then generate more free radicals.However, at excessive current densities, the scavenging phenomena may occur, leading to low removal efficiency and high required electrical energy [59].In addition, the excessive current density can adversely affect the ability of electrodes for oxidising organic matters and ammonia.Lomocso and Baranova [84] studied the effect of current density of ammonia removal through electrochemical process with Pt electrodes, and by increasing current density up to optimum value, the ammonia removal increased, but after that the ammonia removal efficiency decreased due to the blocking active sites, preventing ammonia electro-oxidation.

Activated carbon persulfate oxidation (APO)
Recently, AC has been introduced as a viable activator for PS due to the presence of the oxygen functional groups on its surface [20].Equations 35 and 36 describe the mechanism of PS activation through activated carbon as thecatalyst [85].In addition to sulphate radicals which can oxidise nitrogen species, ammonia and other nitrogen-containing matters such as nitrite and nitrate can be absorbed on the AC surface [86][87][88].
Xu et al. [89] reported that ACwas effective for the catalytic wet air oxidation (CWAO) at the presence of PS to degrade organic matters.The concentration of AC has an important role in the performance of APO process as the high concentration of AC will increase the active sites, which will be accompanied with absorbing more organic matters and increasing in the removal efficiency due to the enhancement sulfate radicals [85].However, at the excessive concentrations, the removal efficiency will increase slowly, and thus the operating costs including the regeneration of AC or replacing them with new ones will rise [90].In addition, the excessive concentrations of PS may occupy AC surface, leading to the reduction in the absorption capacity.In other words, PS may compete with organic matters to be absorbed by AC [91].The high removal efficiency of organic matters by APO process has been reported at acidic pH [89,90].Theoretically, if pH of the solution is less than pH PZC (PZC: point of zero charge), the surface of AC will become positively charged; otherwise, the surface of AC will become negatively charged [92].When the pH is less than PZC point, most number of anions may be absorbed on the surface of AC, resulting in the high concentration of sulphate radicals in the wastewater.Furthermore, as most of the organic matters are negatively charged, the performance of absorption process by AC will be enhanced as a result of the increase in the attractive electrostatic forces between the organic matters and AC surface [93].In contrast, by increasing the pH of the solution above pH pzc , the surface of AC is positively charged, which causes repulsive electrostatic forces between AC surface and ammonium, leading to the decrease in removal efficiency [94].

Ozone persulphate oxidation (OPO)
Ozonation process can degrade a wide range of organic matters through direct oxidation by the ozone molecules (O 3 ) and its derivatives, especially free radicals such as hydroxyl radicals and sulphate radicals, which are stronger and more selective than O 3 [95,96].OPO can quickly degrade pollutants, as the optimum reaction time of OPO for LL treatment varies from 60 to 210 minutes [80,81,97].In O 3 /PS process, sulphate radicals along with other oxidants including HO are formed [98].It is reported that the removal efficiency of OPO is more than O 3 /H 2 O 2 process [99].In addition, O 3 molecules are able to directly oxidize ammonia to nitrate according to Equation 37 [100].Amr et al. [80] compared the performance of ozone and OPO for a stabilized solid waste leachate, and the total COD (COD total ) removal efficiency by OPO and O 3 reached to 72% and 15%, respectively.The appropriate dosage of ozone varies from 30 to 80 g/m 3 , which mostly depends on the concentration of pollutants in LLW [80,101].Regardless of energy consumption, increasing ozone dosage leads to the generation of excessive free radicals which may emerge scavenging phenomena, and then reduce removal efficiency [99].
In another study, Amr et al. [81] compared the performance of O 3 /PS with PS alone, O 3 alone and O 3 followed by PS, and the O 3 /PS process with COD, colour and ammonia removal of 72%, 93%, and 55%, respectively, was more effective than other processes.Soubh and Mokhtarani [97] assessed the efficiency of O 3 /PS for the treatment of LL under different levels of pH, persulfate dosage, and reaction time, and the removal efficiency of COD and color reached 87% and 85%, respectively.In alkaline pH, 9-10, the efficiency of COD and ammonia removal by OPO is significant, asozone molecules are able to produce more hydroxyl radicals at alkaline pH [80,81,99].

Radiation persulphate oxidation (RPO)
RPO processes include ultraviolet (UVPO), microwave (MWPO) and ultrasonic (USPO) which can provide efficient energy for the activation of PS according to Equation 38.In Table S2, some works on RPO processes for the treatment of LL have been presented.

MWPO
MWPO can be considered as a reliable alternative for conventional methods such as HPO due to its shorter reaction time and lower energy consumption, as the reaction time of MWPO for the treatment of LL varies between 10 and 30 min [102][103][104].
According to MW mechanism, the energy is conveyed through radiation which is then converted into heat due to the rotation of the H 2 O [41,105].MW can remove ammonia through thermal and non-thermal mechanisms.The thermal effects of MW comes from the heat generated by the energy of microwave radiations absorbed by water and other polar molecules [41].Hence, the mechanism for the ammonia removal by MW is proposed based on the evaporation of NH 3 by MW radiation [106].On the other hand, the nonthermal effects of MWon ammonia removal are related to the polarisation of NH 3 by MW radiation [41].However, the application of MW radiation alone cannot break the bonds of all contaminants, especially non-degradable substances present in LLW.Hence, MW is usually incorporated in powerful oxidants like PS to produce free radicals for the degradation of organic and inorganic compounds present in LL [107].Nonetheless, according to some studies, it has been proved that the performance of MW in ammonia removal is more than MW/PS.Tripathy and Kumar [102] showed that MW with 97% of ammonia removal efficiency after 10 minutes was better than MW/PS with 93% removal efficiency, due to the reduction in pH after adding PS.For the COD removal from LL, Zhang et al. [103] indicated that there was no COD decrease within 50 min by MW alone, while the COD removal efficiency by MW/PS and MW/PMS was 97.3% and 80.2%, respectively.In one study, Chen et al. [108] reported that MW/PS was able to degrade the contaminants in LL under a wide range of pH values especially at acidic pH.Similarly, Chou et al. [104] showed that total organic compounds (TOC) reduction for LL at pH of 3, 5, 7, and 9 were slightly changed.MW power is considered as important another parameter in MW/PS.It was reported that an increase in MW output power may enhanced the removal rate of organics in LLW, but an excessive MW output power is more likely to improve k obs of MW/ PS, without having significant removal efficiency [108,109].Chou et al. [104] reported that the maximum TOC reduction was at 550 W and PS concentration of 4762 mg/L, by which the TOC concentration decreased from 57.7 to 11.9 mg/L, but at higher MW power setting (775 W), the reaction rate decreased due to the rapid decay of PS and scavenging termination of free radicals.

UVPO
The UV can generate sulphate radicals through breaking the O-O bonds of PS [110,111].Ishak et al. [112] showed that the COD and TOC removal by C-F/UVPO process were 90.9% and 88.4%, which was almost similar to C-F/UV followed by PMS, but the maximum ammonia removal efficiency reached 4.92% through UVPO.In one study which was conducted by El Mrabet et al. [113], the combination of UV with MPO increased COD removal from LL up to 10% so that the COD under 60 min of UV-A irradiation reached 94%.
However, Jiang et al. [114] reported that by increasing pH from 3.02 to 8.96, the COD removal of UV/PMS for LL increased to 70.6%, while the COD removal efficiency decreased to 54.38% at alkaline pH.Implementing other efficient oxidants in combination with UPVO can compensate for the lack of ammonia removal efficiency by UPVO.Poblete et al [115] indicated that the combination of UV solar /O 3 /H 2 O 2 /S 2 O 8 −2 followed by natural zeolite (post-treatment) had COD and ammonia removal efficiency of 36% and 99%, respectively.The UV wavelength plays an essential role in PS activation through UVPO process.In some studies, the wavelength of 254 nm has been reported as optimal wavelength [116][117][118].By coupling other processes with UVPO, the wavelength may be reduced, as El Mrabet et al. [113] indicated that adding Fe 2+ to UVPO significantly decreased the absorbance ranging from 250 to 300 nm.

USPO
Like MW, US can transfer energy to molecules through radiation.Yang et al. [119] reported that TOC removal efficiency for LL by USPO was 77.3%.High temperatures result in the increase of reaction rate of USPO [120].The appropriate temperature of USPO for LL treatment was reported as 70°C [119].Generally, the optimum PS concentration for USPO varies between 1500 and 5000 mg/L.Some studies have indicated that acidic pH conditions, from 3 to 6, generate more sulphate radicals in USPO, causing more degradation of organic matters [121,122].It is found that the removal efficiency using USPO for the treatment of LL is more effective at acidic pH ranging from 3.5 to 4 [119,123].Tripathy et al. [123] showed that US alone could increase the biodegradability ratio of LL from 0.036 to 0.142 at US power of 150 W and COD removal efficiency reached 67%, while it increased up to 86% through USPO.Nonetheless, among various processes, the US/H 2 O 2 by COD removal efficiency of 93% had higher performance than US alone, coagulation-US, and USPO that their removal efficiencies reached 67%, 78%, and 86%, respectively.The extreme increase of ultrasound power may end up the cavitation phenomena in which gas bubbles produced during oxidation process cause a significant decrease in the number of sulphate radicals due to interruption of the reaction between US and PS [121,124].In addition, excessive US power will increase operating costs [123].The proper US power for the treatment of LL has been reported at 300 Watt [119].

Drawbacks and thefuture challenges for the use of SR-AOP technologies
SR-AOPs have indicated promising results for the removal of COD and ammonia.But several critical drawbacks appear to make SR-AOPs impractical for LL treatment.The first main drawback is attributed to high energy demand [125].Although there is no report about the amount of energy required for TPO, it demands high level of energy in the form of heat for the activation of PS [15].Nonetheless, there are some strategies which may be adopted to provide a major part of energy for decaying PS, causing considerable reduction in the operating costs.It is suggested that anaerobic digestion (AD) along with the biogas obtained during digestion process can be able to provide the required heat and energy for TPO or other SR-AOPs (Figure S3) [15,126].On the other hand, a high PS concentration is required for the removal of high COD and ammonia levels.Generally, the COD/PS ratio varies between from 2 to 5, and in some cases this ratio exceeds over 10.Moreover, the pH adjustment make the SR-AOPs more sophisticated as the preferable pH for SR-AOPs ranges from 3 to 6 by which lots of chemicals and operating facilities such as pumps, tanks, pipes, monitoring devices, and mixers are required to set the pH at the beginning and end of SR-AOPs.In some SR-AOPs like MPO, a considerable amounts of chemical agents are required, which will be accompanied with considerable volume of chemical waste as residue of transition metals, resulting in secondary pollution and increase in the disposal or recovery-related costs [127].Another drawback is related to the production of high TDS as a result of adding PS.Using some conventional physical and chemical pretreatment methods such as C-F can somehow alleviate the mentioned problems.Gholami et al. [14] indicated that the use of C-F as a pretreatment for MPO reduced the sludge production and the amount of PS up to 66% and 80%, respectively.
The major defect of APO is that the catalytic capacity of AC is so limited.Hence, during APO, the regeneration methods, including thermal treatment or solvent extraction, are required to recover AC capacity to sufficiently react with PS, resulting in the high operating costs and the decrease in the reusability of AC in each regeneration cycle [20,128,129].The removal efficiency begins to decrease by regeneration treatment after four times [87,89,90,129].The defects of EPO are associated with the high energy consumption, cost of electrodes, and the fouling of their surface.The initial capital investment needed for EPO based on boron-doped diamond (BDD) is higher than conventional AOPs like Fenton process [130].Similarly, the operating costs and energy consumption of RPO and OPO are high due to the generation of O 3 or UV/MW/US radiation [131].Using the combination of various SR-AOPs may significantly alleviate the current difficulties.It is reported that the radiation time can be reduced by a combination of MPO with MWPO, resulting in lower energy consumption and chemical agents with higher removal efficiency [132].The penetration depth of radiation, which is regarded as a major obstacle for RPO process due to the high level of turbidity in LL, may be remarkably addressed by using appropriate pretreatment methods [133].Ishak et al. [112] observed that the C-F pretreatment coupled with USPO was useful method for treatment of LL.Asaithambi et al. [134] showed that UV/Fe 2+ /H 2 O 2 process had higher removal efficiency with lower EEo than UV, UV/H 2 O 2 , UV/Fe 2+ process.
Some viable energy source such as biogas or solar energy can be considered as a remarkable approach for the generation of sustainable energy for SR-AOPs.The use of solar power (UV solar ) instead of UV lamps can be considered as a viable source of energy for UVPO or other oxidation processes which consume a large amount of electricity [135].

The comparison of SR-AOPs with HR-AOPs
There are a few scientific studies for the comparison between sulphate and hydroxyl radicals for the treatment of LL.Generally, hydroxyl and sulphate radicals share similar features [15].One of the main differences between sulphate and hydroxyl radicals is attributed to the production of sulphate radicals at a wide pH range of 2.0-11.0,which was more effective than hydroxyl radicals (pH 2-6) [53].Chen et al. [108] reported that hydroxyl radicals are able to degrade organic matters from LL under acidic conditions, whereas sulphate radicals can be more effective in a wide range of pHs.In Figure S4, the comparison of sulphate radicals-based and hydroxyl radicals-based technologies for the treatment of LL are presented, and the available results from scientific reports showed that both of them have outstanding performance for COD removal.However, SR-AOPs have relatively better performance in some cases.Based on the different nature of these two radicals, SO 4 •-can efficiently degrade humic acid-like (HAL) substances, whereas •OH shows better performance in degradation of fulvic acid-like (FAL) substances [108].In another study, Chen et al. [109] reported that sulphate radicals could significantly degrade the refractory organic matters such as polycyclic aromatics and polyphenols from LL in comparison with hydroxyl radicals.The distinguishing feature of sulphate radicals is attributed to its ability for elimination of ammonia from LL.According to the ammonia removal efficiency, almost all of SR-AOPs showed promising performance (ammonia removal from 70% to 100%).Deng & Ezyske [15] showed the ammonia removal efficiency by hydroxyl radicals was about zero, while the ammonia removal reached 79% by sulphate radicals.It has been proved that sulphate radicals have more degradation constant rate for treatment of organic matters than hydroxyl radicals [136].Ahmed et al. [137]138 indicated that the constant rate of sulphate radicals was about ten times more than hydroxyl radicals.In contrast to hydroxyl radical processes, SR-AOPs consume lower concentration of catalyst or oxidant and thus produce low chemical sludge.However, the price of PS is relatively higher than H 2 O 2 [139].In addition, it was reported that the sulphate-radical processes consumed less energy.Chen et al. [108] revealed that the energy saved by the MW/PS process was about 614.2 kWh per unit volume of effluent compared to the MW/H 2 O 2 process.Similarly, Varank et al. [83] revealed that electrochemical process based on sulphate radicals with 71.4% outperformed hydroxyl radicals with 60.8%.In addition, they reported that the total cost of electro-Fenton process with 5.0 €/m 3 was higher than EPO process with 2.8 €/m 3 .

Conclusion and future research and engineering needs
The conventional treatment methods such as biological and physical processes are not able to remove hazardous materials significantly from LL, whereas SR-AOPs have shown great potential in the organic and nitrogen compounds removal.In some studies, the ammonia removal efficiency reached 100%, which implies the distinctive feature of sulphate radicals in comparison to hydroxyl radical-based processes.Nonactivated PSslowly reacts with contaminants, but by implementing some activation methods including TPO, MPO, EPO, APO, RPO, HPO, and OPO sulphate radicals with high oxidation potential are produced, accelerating degradation rate.In the present review, sulphate radical activation methods and their performance for treatment of LL have been studied and evaluated.In addition, the influence of effective parameters and their optimum range for each SR-AOPs have been studied.Among SR-AOPs, thermal activation is considered as a reliable process with desirable simplicity and high COD and ammonia removal efficiency, but it requires considerable energy consumption, which seems to be a major obstacle for the application of this method for PS activation.On the other hand, the MPO process is known as a rapid method for the elimination of contaminants and PS activation.Nevertheless, this process usually requires extremely acidic pH to maximise the removal efficiency, resulting in a high amount of chemical reagents consumption and high chemical sludge production.Hence, new approaches such as APO, EPO, RPO, OPO, and HPO can be effective both operationally and financially.However, given the relevant challenges for SR-AOPs, future studies for the treatment of LL should be dedicated to: (1) Determining strategies to provide sustainable energy source for SR-AOPs via biogas or other green energy sources, (2) Improving the performance of SR-AOPs both financially and operationally by understanding of effective parameters and removal mechanisms occurring during oxidation process, (3) Assessing the use of complementary methods to increase the removal efficiency of SR-AOPs and minimise the technical and operational difficulties, (4) Improving the removal of ammonia, nitrate, and nitrite by SR-AOPs.