Titanium dioxide nanoparticles (TiO2-NP) present in wastewater effluent are discharged into freshwater and saltwater (i.e., marine) systems. TiO2-NP can be solar-driven photoactivated by ultraviolet (UV)-light producing reactive oxygen species including hydroxyl radicals (·OH). ·OH are non-selective and react with a broad range of species in water. In other studies, photoactivation of TiO2-NP has been correlated with oxidative stress and ecotoxicological impacts on plant and animal biota. This study examined the photoactivation of TiO2-NP in freshwater and saltwater systems, and contrasted the oxidation potential in both systems using methylene blue (MB) as a reaction probe. Maximum MB loss (51.9%, n=4; 95% confidence interval 49.4-54.5) was measured in salt-free, deionized water where ·OH scavenging was negligible; minimum MB loss (1%) was measured in saltwater due to significant ·OH scavenging, indicating the inverse correlation between MB loss and radical scavenging. A kinetic analysis of scavenging by seawater constituents indicated Cl- had the greatest impact due to high concentration and high reaction rate constant. Significant loss of MB occurred in the presence of Br- relative to other less aggressive scavengers present in seawater (i.e., HCO3-, HSO4-). This result is consistent with the formation of Bromate, a strong oxidant that subsequently reacts with MB. In freshwater samples collected from different water bodies in Oklahoma (n=12), the average MB loss was 13.4%. Greater MB loss in freshwater systems relative to marine systems was due to lower ·OH scavenging by various water quality parameters. Overall, TiO2-NP photoactivation in freshwater systems has the potential to cause greater oxidative stress and ecotoxicological impacts than in marine systems where ·OH scavenging is a dominant reaction.

Groundwater treatment of recalcitrant fumigant pesticides (1,2-dibromo-3-chloropropane (DBCP), 1,2-dibromo-ethane (EDB), 1,2-dichloropropane (DCP), and 1,2,3-trichloropropane (TCP)) often involves a pump and treat system with granular activated carbon (GAC). A novel and promising method of treating the pesticide-spent GAC is based on alkaline hydrolysis, a well-understood abiotic transformation mechanism, that offers a potentially greener approach to conventional thermal regeneration. Here, alkaline hydrolysis of these pesticide chemicals was evaluated under homogeneous (aqueous), and heterogeneous (pesticide spent-GAC) conditions involving bituminous-and coconut-based GAC. Aqueous treatment occurred at elevated pH (pH 12.0-12.4) and the pesticide rate of hydrolysis transformation was first-order (DBCP >> TCP >> EDB >> DCP). Significant pesticide loss (94.95-99.98%) was achieved in both types of GAC (pH 12.0-12.4; 30 d). GAC suspensions held (5 d) at pH 11.0, 12.0, and 12.6, resulted in the DBCP loss of 74%, 89%, and 99%, respectively. The pH dependency of DBCP hydrolysis underscores the correlation between alkaline conditions, aggressive hydrolysis treatment, and reac-tion time for engineered systems. The estimated time (4-8 min) for full OH- intraparticle diffusion into the GAC from bulk solution was much less than the pesticide hydrolysis half-lives indicating that alkaline hydrolysis treatment of pesticides in GAC was reaction rate limited. Rapid small scale column tests demonstrated that the post-treatment (i.e., base hydrolysis) impact on adsorptive characteristics of the GAC was limited.

For the first time, the fate of radicals generated in heterogeneous chemical oxidation treatment systems has been accounted for and used to assess treatment performance in three reaction compartments; reaction with the target compound, rhodamine B (RhB), the aqueous phase scavengers, and the solid phase scavengers. Radicals formed during the ultra-violet (UV) activation of hydrogen peroxide (H2O2) (UV-AHP) and persulfate (S2O82-) (UV-APS) include hydroxyl (•OH) and sulfate radicals (SO4•-), respectively. •OH and SO4•-, used in oxidation treatment systems to degrade a broad spectrum of environmental contaminants, may also react with non-target chemical species (scavengers) that limit treatment efficiency. UV-AHP and UV-APS treatment systems were amended with solid phase alumina to assess scavenging by solid surfaces. The overall rate of reaction and rate of radical scavenging was greater for •OH than SO4•-. Scavenging by dissolved constituents was dominated by the oxidant used (H2O2, S2O82-); and the rate of radical scavenging by alumina was greater than the rate of RhB oxidation in all cases. Treatment efficiency was lower in the UV-AHP than in the UV-APS treatment system and was attributed to greater aqueous and solid phase scavenging rates. The cost of commercially available H2O2 ($0.031 mol-1) and PS ($0.24 mol-1) was used in conjunction with the overall treatment efficiency to assess specific cost of treatment. The specific cost to treat the probe compound with UV-AHP was greater than UV-APS and was attributed to the much lower treatment efficiency with UV-AHP. The much-desired high reaction rate constants between •OH and environmental contaminants, relative to SO4•-, may come at the cost of greater combined scavenging rates, and consequently lower treatment efficiency.

Activated persulfate (PS) is a common method used to generate sulfate radicals (SO4•-), a powerful oxidant capable of degrading a broad array of environmental contaminants. The reaction of SO4•- with nontarget species (i.e., scavenging) contributes significantly to treatment inefficiency. Radical scavenging in this manner has been quantified for nontarget chemical species in the aqueous phase but has never been quantified for solid phase media. Kinetic analysis and laboratory methods were developed to quantify the SO4•- scavenging rate constant (k≡S) for alumina, a naturally occurring mineral in soil and aquifer materials. SO4•- were generated in UV and thermally activated persulfate (UV-APS, T-APS) batch systems, and the loss of rhodamine B (RhB) served as an indicator of SO4•- activity. k≡S for alumina was 2.42 × 104 and 2.03 × 104 m-2 s-1 for UV-APS and T-APS oxidative treatment systems, respectively. At [alumina] >5 g L-1, the reaction of SO4•- with solid phase media increased over the aqueous phase reactions with RhB and aqueous scavengers. SO4•- scavenging by solid surfaces was orders of magnitude greater than the reaction with the target compound and scavengers in the aqueous phase, underscoring the significant role of solid surfaces in scavenging SO4•-.

The comments (Kopinke, 2020) do not accurately represent the experimental conditions, parameter values, and conceptual model used in our study (Rusevova and Huling, 2020), leading to a misinterpretation of the results. The kinetic analysis used to determine the surface scavenging rate constant (kS) involved competition kinetics between the probe and the mineral surfaces (Rusevova and Huling, 2020); similar competition kinetic methods have been used regarding other widely reported rate constants (Buxton et al., 1988; Dorfman and Adams, 1973). Assuming mass transport limitations prevented ·OH reaction at mineral surfaces, as proposed in the comments, estimates of kS would be very small, or 0. However, the addition of solid phase media was found to effectively and consistently compete with the probe. The propensity of data, supporting data, and high degree of quality assurance and quality control measures provide multiple lines of evidence indicating a high level of certainty that ·OH scavenging occurred by solid surfaces.

Advanced oxidation treatment processes used in various applications to treat contaminated soil, water, and groundwater involve powerful radical intermediates, including hydroxyl radicals (center dot OH). Inefficiency in center dot OH-driven treatment systems involves scavenging reactions where center dot OH react with non-target species in the aqueous and solid phases. Here, center dot OH were generated in iron (Fe)- and UV-activated hydrogen peroxide (Fe-AHP, UV-AHP) systems where the loss of rhodamine B served as a quantitative metric for center dot OH activity. Kinetic analysis methods were developed to estimate the specific center dot OH surface scavenging rate constant (k( S)). In the Fe-AHP system, k( S) for silica (2.85 x 10(6) 1 /m(2) x s) and alumina (3.92 x 10(6) 1/ m(2) x s) were similar. In the UV-AHP system, estimates of k( S) for silica (4.50 x 10(6) 1/m(2) x s) and alumina (7.45 x 10(6) 1/m(2) x s) were higher. k( S) for montmorillonite (MMT) in the UV-AHP system was <= 4.22 x 10(5) 1/m(2) x s. Overall, k( S),(silica) - k( S), alumina > k( S),(MMT) indicating k( S) is mineral specific. Radical scavenging was dominated by surface scavenging at 10-50 g/L silica, alumina, or MMT, in both Fe-AHP and UV-AHP systems. The experimentally-derived surface center dot OH scavenging rate constants were extended to in-situ chemical oxidation (ISCO) treatment conditions to contrast center dot OH reaction rates with contaminant and aqueous phase reactants found in aquifer systems. center dot OH reaction was dominated by solid surfaces comprised of silica, alumina, and montmorillonite minerals relative to center dot OH reaction with trichloroethylene, the target compound, and H2O2, a well-documented radical scavenger. These results indicate that solid mineral surfaces play a key role in limiting the degradation rate of contaminants found in soil and groundwater, and the overall treatment efficiency in ISCO systems. The aggressive center dot OH scavenging measured was partially attributed to the relative abundance of scavenging sites on mineral surfaces. Published by Elsevier Ltd.

Raw materials, activation methods, and post-activation treatment used in manufacturing granular activated carbon (GAC) results in a spectrum of physicochemical characteristics that potentially impact the adsorption oxidation treatment process. A comprehensive study is lacking that assesses the effect of GAC characteristics on adsorption oxidation treatment of contaminant spent-GAC. Consequently, it is inherently assumed the treatment process is GAC-independent. Here, GACs (n = 31) were characterized and used in the hydrogen peroxide (H2O2)-based adsorption oxidation treatment of 2-chlorophenol (2CP)-spent GAC. The GACs exhibited a range in surface area, pore volume distribution, metals content, surface functionality, and H2O2 reaction. Chloride recovery, the treatment metric for 2CP oxidation, indicated a wide range in oxidation (0-49.2%) where bituminous- and wood-based GAC performed best. A selected subset of GACs (n = 12), amended with iron, methyl tert-butyl ether (MTBE), and H2O2, exhibited a range in oxidative treatment (1.1-57.9%). Correlations were established between GAC surface functionality, H2O2 reactivity, adsorption, and MTBE oxidation indicating multiple parameters play a collective and compounding role. The order of GACs successfully used in the treatment process is bituminous-based coal > wood > coconut > peat. Results showed adsorption oxidation treatment is GAC-dependent, and therefore, GAC selection is a key factor in the success of this technology.

A method for the remediation of polycyclic aromatic hydrocarbons (PAHs) contaminated soils was proposed involving a combination of surfactant-aided soil washing and chemical oxidation by activated persulfate (SP). In this study, Triton X-100 (TX-100) and SP was applied to the soil, either concurrently or sequentially. Results indicated that surfactant followed by amendment with a solution of SP, TX-100 + SP(1), was most effective in decreasing PAHs concentrations in a sandy loam soil (SS) and a silty clay soil (NS) from 1220 mg/kg and 2730 mg.kg(-1) to 414 mg.kg(-1) and 180 mg.kg(-1), respectively. Compared with extraction alone and oxidation alone, TX-100 + SP(1) increased the removal of PAHs by 10-20%. TX-100 improved the degradation of 3-4 ring PAHs (M-PAHs) and 5-6 ring PAHs (H-PAHs) in SS, by approximately 8%-11%. The oxygenated polycyclic aromatic hydrocarbons (oxy-PAHs) including furans and xanthene exhibited greater reductions in soil when amended with the TX-100 and SP, than under TX-100 extraction or SP oxidation alone. Overall, increased removal of PAHs in contaminated soil can occur through simultaneous application of TX-100 and SP, relative to the sole use of TX-100 or SP. The sequential combination of surfactant and oxidant was most effective for the elimination of PAHs, especially for M-PAHs and H-PAHs in sandy loam contaminated soil. (C) 2018 Elsevier B.V. All rights reserved.

Hydroxylamine (NH2OH) undergoes biotic and abiotic transformation processes in soil, producing nitrous oxide gas (N2O(g)). Little is known about the magnitude of the abiotic chemical processes in the global N cycle, and the role of abiotic nitrification is still neglected inmost current nitrogen trace gas studies. The abiotic fate of NH2OH in soil systems is often focused on transition metals including manganese (Mn) and iron (Fe), and empirical correlations of nitrogen residual species including nitrite (NO2-), nitrate (NO3-), and N2O(g). In this study, abiotic NH2OH nitrification by well-characterized manganese (Mn)- and iron (Fe)-bearing minerals (pyrolusite, amorphous MnO2(s), goethite, amorphous FeOOH(s)) was investigated. A nitrogen mass balance analysis involving NH2OH, and the abiotic nitrification residuals, N2O(g), N2O(aq), NO2-, NO3-, was used, and specific reactions and mechanisms were investigated. Rapid and complete NH2OH nitrification occurred (4-5 h) in the presence of pyrolusite and amorphous MnO2(s), achieving a 95-96% mass balance of N byproducts. Conversely, NH2OH nitrification was considerably slower by amorphous FeOOH(s) (14.5%) and goethite (1.1%). Direct reactions between the Mn- and Fe-bearing mineral species and NO2- and NO3- were not detected. Brunauer-Emmett-Teller surface area and energy dispersive X-ray measurements for elemental composition were used to determine the specific concentrations of Mn and Fe. Despite similar specific concentrations of Mn and Fe in crystalline and amorphous minerals, the rate of NH2OH nitrification was much greater in the Mn-bearing minerals. Results underscore the intrinsically faster NH2OH nitrification by Mn minerals than Fe minerals. (C) 2018 Published by Elsevier B.V.

Contaminant rebound and low contaminant removal are reported more frequently with in situ chemical oxidation than other in situ technologies. Although there are multiple causes for these results, a critical analysis indicates that low oxidant volume delivery is a key issue. The volume of oxidant injected is critical and porosity of the aquifer matrix can be used to estimate the pore volume. The total porosity ( q T ) is the volume of voids relative to the total volume of aquifer material. The mobile porosity ( q M ) is the fraction of voids that readily contributes to fluid displacement, and is less than q T leading to smaller estimates of oxidant volume. Injecting low‐oxidant volume may result in inadequate oxidant distribution and postinjection dispersal within the radius of influence, insufficient oxidant contact and oxidant loading, and incomplete treatment; whereas, greater oxidant volume achieves a greater oxidant footprint and may involve risk that the injected oxidant may migrate into nontarget areas and displacement of contaminated groundwater. Design guidelines and recommendations are provided that could help achieve more effective technology deployment, reduce the role of heterogeneities in the subsurface, and result in greater probability the oxidant is delivered to the targeted treatment zone.

Arsenic (As) contamination in drinking water is an epidemic in many areas of the world, especially Eastern Asian countries. Developing affordable and efficient procedures to remove arsenic from drinking water is critical to protect human health. In this study, the oxidation of aquifer solids through the use of sodium permanganate (NaMnO4), hydrogen peroxide (H2O2), and exposure to air, enhanced the adsorption of arsenic to the aquifer material resulting in treatment of the water. NaMnO4 was more effective than H2O2. NaMnO4 was tested at different loading rates (0.5, 1.5, 2.4, 3.4, and 4.9 g NaMnO4/kg aquifer material), and after 30 days contact time, arsenic removal ([As+3]INITIAL = 610 μg/L) was 77%, 88%, 93%, 95%, 97%, respectively, relative to un-oxidized aquifer material. Arsenic removal increased with increasing contact time (30, 60, 90 days) suggesting removal was not reversible under the conditions of these experiments. Oxidative treatment by exposing the aquifer solids to air for 68 days resulted in >99% removal of Arsenic ([As+3]INITIAL = 550 μg/L). Less arsenic removal (38.2%) was measured in the un-oxidized aquifer material. In-situ oxidation of aquifer materials using NaMnO4, or ex-situ oxidation of aquifer materials through exposure to air could be effective in the removal of arsenic in ground water and a potential treatment method to protect human health.

This chapter discusses the effect of irrigation water management and drainage system design and management on drain water quality in arid and semiarid areas. The water quality concerns in irrigated lands have been focused on salinity management in the shallow groundwater in the soil profile and the water supply used for irrigation. The presence of toxic trace elements, such as selenium (Se), boron (B), arsenic, and many others, is one of the major water-quality concerns in arid and semiarid irrigated areas. The load-flow relationship is determined by calculating the mass of salt and plotting the resultant as a function of the discharge. In the computations for the individual sumps, both the load and flow were calculated on a unit area basis and only the drain flow was sampled. In the arid West where irrigation is essential for crop production, drain water quality has been of concern for decades.

The present study suggests a combined adsorption and Fenton oxidation using an acid-treated Fe-amended granular activated carbon (Fe-GAC) for effective removal of bisphenol A in water. When the Fe-GAC adsorbs, and is saturated with BPA in water, Fenton oxidation of BPA occurs in the BPA-spent Fe-GAC for regeneration of the GAC. Particularly this study showed temperature as an effective mean to enhance adsorption, desorption, diffusion and Fenton oxidation associated with the adsorption and Fenton oxidation of BPA on the Fe-GAC. The adsorption rates of BPA onto the Fe-GAC were enhanced with increasing temperature mainly due to increase in diffusion of BPA. The estimated thermodynamic parameters associated with adsorption of BPA indicated that the adsorption of BPA onto the Fe-GAC was a spontaneous, exothermic and physical adsorption process. On the other hand, the molar ratio of [H2O2]:[BPA] (36–108 H2O2/mol BPA) in the Fenton oxidation of the BPA-spent Fe-GAC led to 91–99% removal of BPA with negligible aromatic products and some soluble organic acids. The oxidation rates of BPA and H2O2 during the Fenton oxidation of the BPA-spent Fe-GAC were drastically enhanced by the factor of 2.5 and 5 when the reaction temperature increased from 293 to 331 K, respectively. The comparative analysis of temperature-dependent enhancement in diffusion, desorption and oxidation rates of BPA in Fe-GAC indicated that the BPA oxidation in the Fe-GAC is mainly controlled by diffusive transport of BPA from the Fe-GAC. Besides, the Thiele-modulus analysis clearly supported more significant pore diffusion limitation of H2O2 in the Fe-GAC with increasing temperature and the Fe-GAC particle size.

The extent of PAH transformation, the formation and transformation of reaction byproducts during persulfate oxidation of polycyclic aromatic hydrocarbons (PAHs) in coking plant soil was investigated. Pre-oxidation analyses indicated that oxygen-containing PAHs (oxy-PAHs) existed in the soil. Oxy-PAHs including 1H-phenalen-1-one, 9H-fluoren-9-one, and 1,8-naphthalic anhydride were also produced during persulfate oxidation of PAHs. Concentration of 1,8-naphthalic anhydride at 4h in thermally activated (50°C) persulfate oxidation (TAPO) treatment increased 12.7 times relative to the oxidant-free control. Additionally, the oxy-PAHs originally present and those generated during oxidation can be oxidized by unactivated or thermally activated persulfate oxidation. For example, 9H-fluoren-9-one concentration decreased 99% at 4h in TAPO treatment relative to the control. Thermally activated persulfate resulted in greater oxy-PAHs removal than unactivated persulfate. Overall, both unactivated and thermally activated persulfate oxidation of PAH-contaminated soil reduced PAH mass, and oxidized most of the reaction byproducts. Consequently, this treatment process could limit environmental risk related to the parent compound and associated reaction byproducts.

Analytical artifacts attributed to the bromination of toluene, xylenes, and trimethylbenzenes were found during the heated headspace gas chromatography/mass spectrometry (GC/MS) analysis of aqueous samples. The aqueous samples were produced from Fenton-like chemical oxidation reactions and contained aromatic compounds, hydrogen peroxide (H2O2), and ferric sulfate. Prior to GC/MS headspace analysis, the samples were acidified (pH<2), and sodium chloride was amended to the headspace vial as a matrix modifier. The brominated artifacts were generated during heated headspace analysis. Further, when samples were spiked with a mixture of volatile chlorinated and aromatic compounds (50 µg/L), poor spike recoveries of toluene and xylenes occurred, and in some cases complete loss of trimethylbenzenes and naphthalene resulted. Where poor recovery of aromatic spike compounds occurred, brominated aromatic compounds were found. The only significant source of bromine in the reaction scheme is the bromide typically present (<0.01% w/w) in the sodium chloride amended to the samples. Conversely, brominated artifacts were absent when a buffered salt mixture composed of sodium chloride and potassium phosphate dibasic/monobasic was used as a matrix modifier and raised the sample pH (pH~6). This indicated that the brominated artifacts resulted from the reaction of the aromatic compounds with BrCl, which was formed by the reaction of H2O2, chloride, and bromide under acidic conditions. An alternative matrix modifier salt is recommended that prevents the bromination reaction and avoids these deleterious effects on sample integrity during headspace analysis.

The influence of persulfate activation methods on polycyclic aromatic hydrocarbons (PAHs) degradation was investigated and included thermal, citrate chelated iron, and alkaline, and a hydrogen peroxide (H2O2)-persulfate binary mixture. Thermal activation (60 degrees C) resulted in the highest removal of PAHs (99.1%) and persulfate consumption during thermal activation varied (0.45-1.38g/kg soil). Persulfate consumption (0.91-1.22 g/kg soil) and PAHs removal (73.3-82.9%) varied using citrate chelated iron. No significant differences in oxidant consumption and PAH removal was measured in the H2O2-persulfate binary mixture and alkaline activated treatment systems, relative to the unactivated control. Greater removal of high molecular weight PAHs was measured with persulfate activation. Electron spin resonance spectra indicated the presence of hydroxyl radicals in thermally activated systems; weak hydroxyl radical activity in the H2O2 persulfate system; and superoxide radicals were predominant in alkaline activated systems. Differences in oxidative ability of the activated persulfate were related to different radicals generated during activation. (c) 2013 Elsevier B.V. All rights reserved.

Granular activated carbon (GAC) can remove trace organic pollutants and natural organic matter (NOM) from industrial and municipal waters. This paper evaluates an iron nanocatalyst approach, based on Fenton-like oxidation reactions, to regenerate spent GAC within a packed bed configuration after saturation by organic compounds. Specifically, we focus on regenerating GAC packed beds equilibrated with varying influent concentrations of phenol, a model organic compound. Iron nanocatalysts were synthesized using ferric chloride, a chemical already used as a coagulant at municipal WTPs, and reacted with hydrogen peroxide (H(2)O(2)) for the purpose of in-situ regeneration. Up to 95% of phenol adsorption capacity was regenerated for GAC equilibrated with 1000 mg/L of phenol. Using this technique, at least four adsorption-regeneration cycles can be performed sequentially for the same batch of GAC with fresh iron nanocatalysts while achieving a regeneration efficiency of 90 ± 5% between each loading. Moreover, the iron nanocatalyst can be recovered and reused multiple times. Lower initial adsorbate concentrations (10-500 mg/L) resulted in a slightly lower saturated adsorbent-phase concentration of phenol and lower regeneration efficiencies (72 ± 5%). Additionally, this catalytic in-situ regeneration was applied to GAC saturated by NOM. A slightly lower regeneration efficiency (60%) was observed for the Suwannee River NOM adsorption capacity of GAC. The next step is validation in a pilot-scale test that applies this regeneration technique to a GAC adsorber employed in NOM removal.

Chemical oxidation regeneration of granular activated carbon (GAC) is a developing technology that can be carried out utilizing thermally-activated persulfate. During chemical regeneration of GAC, aggressive oxidative conditions lead to high acidity (pH<2) and the accumulation of sodium persulfate residuals in the GAC. In this study, we investigated the impact of chemical oxidation on the sorption characteristics of methyl-tert butyl ether (MTBE) in GAC. Loss of MTBE sorption was measured in thermally-activated persulfate regenerated GAC. The accumulation of sulfur was partially responsible for the blockage of sorption sites, but sorption loss was amplified under oxidizing and acidic conditions and attributed to the formation of acidic surface oxides and enhanced electrostatic attraction and accumulation of SO(4)(2-) in GAC. Raising the pH in the GAC slurry resulted in the removal of the residual sulfate and improved MTBE sorption indicating that the mechanisms responsible for MTBE sorption loss were reversible. These results establish baseline conditions and parameters that can be used to optimize pilot- and full-scale deployment of thermally-activated persulfate regeneration of GAC.

The goal of this study was to assess the oxidation of NAPL in soil, 30% of which were composed of chlorinated ethanes and ethenes, using catalyzed hydrogen peroxide (CHP), activated persulfate (AP), and H(2)O(2)-persulfate (HP) co-amendment systems. Citrate, a buffer and iron ligand, was amended to the treatment system to enhance oxidative treatment. Four activation/catalysis methods were employed: (1) oxidant only, (2) oxidant-citrate, (3) oxidant-iron(II), and (4) oxidant-citrate-iron(II). The NAPL treatment effectiveness was the greatest in the CHP reactions, the second in HP, and the third in AP. The effective activation and catalysis methods depended on the oxidant types; oxidant only for CHP and HP and oxidant-citrate-iron for AP. The treatability trend of chlorinated ethanes and ethenes in the soil mixture was as follows: trichloroethene > tetrachloroethene > dichloroethane > trichloroethane > tetrachloroethane. A significant fraction of persulfate remained in the oxidation systems after the 2-day reaction period, especially in the citrate-iron(II) AP. In general, oxidation systems that included citrate maintained a post-treatment pH in the range of 7-9. A final pH of AP oxidation systems was acidic (pH 2-3), where a molar ratio of citrate-iron(II) was less than 1.8 and where no citrate was amended.