Mechanism Of Advanced Oxidation Processes On Treating Organic Matters In High Salinity Reverse Osmosis Concentrate

Reverse osmosis(RO)concentrate with high salinity and organic contaminants was simultaneously generated in wastewater reclamation and seawater desalination and may pose potential threat to environment without appropriate treatment.In this study,potential of coagulation/activated carbon adsorption combined with UV/H₂O₂ and ozonation for removing organic matter from RO concentrate was systematically investigated.Meanwhile,targeting effects of hydroxyl radical for organics degradation during advanced oxidation processes(AOPS)was also evaluated.Two hybrid strategies were proposed to treat the RO concentrate and yielded favorable efficiencies.Fractionation results shows that hydrophobic organics were the majority of RO concentrate with percentage of 70.2%.The percentage of the five organic fractions has the order HoA(33.5%)>Hi M(29.8%)>Ho N(26.5%)>WHoA(8.5%)>Ho B(1.7%).14 typical organics with molecular weight from 92?450 g mol^-1 were detected in RO concentrate by GC-MS method,which contains 12 aromatic organics,1long-chain alkane and 1 polycyclic enol.To investigate the oxidation mechnism of DOM in RO concentrate during UV/H₂O₂ process,a kinetic model involving all possible reactions was established.With high concentration of chloride in alkaline RO concentrate,chloride may act as catalyst-like ion for hydroxyl radical circulation and boosted the optimal p H for oxidation to basic condition.After 6.0 oxidation at a dosage of 10 mmol L^-1 H₂O₂,the removal efficiencies of DOC,COD and UV₂₅₄ were 87.4%?63.4%and 89.1%,respectively.The relationship between the observed pseudo-first-order constant and H₂O₂ dosage was as follows:k_obs=0.116×[H₂O₂]^0.582 and the experimental data agreed well with this model.Fractionation results showed that a portion of hydrophobic organics were first transformed into hydrophilic fractions prior to final mineralization.The percentage of organics reduction has the order WHoA(90.8%)>Ho B(89.7%)>HoA(89.1%)>Ho N(88.18%)>Hi M(83.8%).Ozonation is an effective method to degrade DOM in RO concentrate.In acidic condition,efficiency of direct oxidation by ozone was limited and the removals of DOC,COD and UV₂₅₄were only 4.0%?3.1%and 5.4%,respectively.In bacic condition,degradation process involving ozone and hydroxyl radical oxidation could boost the removals of DOC,COD and UV₂₅₄ to58.2%?41.7%and 74.6%,respectively.Similar to UV/H₂O₂ process,a portion of hydrophobic organics were first transformed into hydrophilic fractions during ozonation.The percentage of the five organic fractions has the order Ho B(82.8%)>WHoA(79.2%)>HoA(74.1%)>Ho N(71.4%)>Hi M(21.1%).During the oxidation process,UV₂₅₄,UV₂₅₀/UV₃₆₅and UV₃₀₀/UV₄₀₀ all decreased gradually,indicating that the relative concentration of humic acid,the molecular weight of organics and the condensation degree all decreased with contact time.Coagulation is efficient in removing organics from RO concentrate.Ferric-based coauglants were better better than the aluminium-based coauglants and their mechamism for DOM removal was charge neutrality.The removals of UV₂₅₄,COD and DOC for FeCl₃ at a dosage of 2.0 mmol L^-1 were 59.0%,24.1%and 39.6%,respectively.Hydrophobic fractions,especially HoA,were sensitive to coagulation,yielding a removal efficiency of 70.5%.The order of DOM reduction through coagulation was HoA(70.5%)>Ho B(29.3%)>Ho N(28.3%)>WHoA(23.9%)>Hi M(20.1%).Coupling coagulation as a pretreatment of UV/H₂O₂,the residual DOC,COD and UV₂₅₄were reduced to 1.90 mg L^-1,40.4 mg L^-1 and 0.015 cm^-1,respectively,which were much better than single coagulation and single UV/H₂O₂ oxidation and meet the demand of the first class discharging standard of our country.The concentration of organic fractions in effluent has the order Hi M(1.223 mg L^-1)>Ho N(0.244 mg L^-1)>WHoA(0.222 mg L^-1)>HoA(0.178 mg L^-1)>Ho B(0.033 mg L^-1).Coupling coagulation as a pretreatment of ozonation,the residual DOC,COD and UV₂₅₄were reduced to 4.40 mg L^-1?77.5 mg L^-1and 0.035 cm^-1,respectively,which were above limit of the first class discharging standard.The concentration of organic fractions in effluent has the order Hi M(2.870 mg L^-1)>WHoA(0.780 mg L^-1)>HoA(0.436 mg L^-1)>Ho N(0.279 mg L^-1)>Ho B(0.035 mg L^-1).Adsorption of DOM from RO concentrate was investigated using four activated carbon in batch adsorption studies as a function of contact time,solution p H and temperature.Freundlich isotherm fitted the experimental data better than Langmuir isotherm,and the pseudo second-order kinetics describe the adsorption behavior well.The adsorption capacities of PAC A and GAC B were 10.775 mg g^-1and 4.948 mg g^-1,respectively.The order of activation energy was GAC B(27.065 k J mol^-1)>PAC A(20.441 k J mol^-1).Fractionation results shows that adsorption favored hydrophobic organics and the order of DOM reduction using GAC was Ho N(99.7%)>HoA(99.6%)>Ho B(87.9%)>WHoA(86.6%)>Hi M(50.2%).Coupling of GAC as a post-treatment with UV/H₂O₂,the residual DOC,COD and UV₂₅₄were reduced to 4.40 mg L^-1?77.5 mg L^-1and 0.035 cm^-1,respectively,which meet the demand of the first class discharging standard.The order of five fractions in effluent was Hi M(2.870 mg L^-1)>WHoA(0.780 mg L^-1)>HoA(0.436 mg L^-1)>Ho N(0.279 mg L^-1)>Ho B(0.035 mg L^-1),which outperformed single GAC adsorption and single UV/H₂O₂process and meet the demand of the first class discharging standard.Coupling of GAC as a post-treatment with ozonation,the residual DOC,COD and UV₂₅₄were reduced to 3.33 mg L^-1?67.6 mg L^-1and 0.032 cm^-1,respectively,which were above limit of the first class discharging standard.The concentration of five fractions in effluent has the order Hi M(2.870 mg L^-1)>WHoA(0.780 mg L^-1)>HoA(0.436 mg L^-1)>Ho N(0.279 mg L^-1)>Ho B(0.035 mg L^-1).