Influence Mechanism Of Coexisiting Constituents In Fracturing Wastewater For Fe/PDS-based Oxidation Of Phthalates

Fracturing wastewater(FWW)is generated from hydraulic fracturing operation of oil and gas wells that contains high and time-fluctuating concentrations of salts,total suspended solid(TSS),and total organic carbon(TOC),which makes it difficult for treatment.The peroxydisulfate(PDS)-induced advanced oxidation processes(AOPs)exhibit higher stability and selectivity for aromatic compounds removal(Compared with hydroxyl radical,sulfate radical has a longer half-life and is more effective on aromatic compounds).Hence,PDS-based AOPs have great potential for the treatment of FWW.However,in the complex matrix of FWW,the potential synergistic effects(or else)of different FWW components(e.g.,ions,organics,and solids)on the PDS-generated radicals still have not been thoroughly studied.In this study,di-(2-ethylhexyl)phthalate(DEHP)in the FWW was chosen as a representative degradation-resistant organic contaminant with high risk to investigate the underlying mechanisms of the Fe²⁺/PDS and Fe⁰/PDS system for FWW treatment.The main objectives of this research were to:(1)illustrate the potential synergistic effects(or else)of different FWW components(i.e.,Cl^-/Br^-,solids,glycol,and polyacrylamide(PAM))on DEHP degradation by the Fe²⁺/PDS system;(2)clarify the potential antagonism effects(or else)of different FWW components(i.e.,Ba²⁺and glycol)on electron transfers on the surface of zero valent iron and DEHP degradation by using the Fe⁰/PDS oxidation system;and(3)compare the enhanced effects of glycol and sulfuric acid in the two oxidation systems(i.e.,Fe²⁺/PDS and Fe⁰/PDS)on DEHP degradation in the real FWW.Besides,this study compares and analyzes the environmental risk and pharmaceutical cost of various treatment processes.The main research results are as follows:(1)An activated oxidation system of Fe²⁺/PDS was established to degrade DEHP in FWW.It was found that the coexisting halogen ions and ethylene glycol had synergistic effect on DEHP degradation in wastewater.The results showed that 41.5%DEHP in the FWW(D-1)(representative to FWW returned to surface 1 day after hydraulic fracturing)could be degraded in 10 min by the Fe²⁺/PDS oxidation system.However,the degradation of DEHP was only 6.5%with the addition of Fe²⁺and PDS in ultra-pure water.Moreover,when chloride ion(313 mM)and bromide ion(1.1 mM)in FWW(D-1)were added,the degradation of DEHP was increased to 12.5%and 28.1%,respectively.After the addition of ethylene glycol,the degradation of DEHP was increased to 45.1%and 42.2%,respectively.Mechanism studies found that direct DEHP oxidation by the sulfate radical and hydroxyl free radical was not obvious by the Fe²⁺activation of PDS,but the coexistence of chlorine/bromine free radicals could more effectively degrade DEHP.Moreover,the coexistence of halide ions with C₂H₄(OH)₂ accelerated the DEHP degradation owing to the generation of^·C₂H₃(OH)₂ that was found capable of reducing inherent Fe³⁺and replenishing Fe²⁺in the Fe²⁺/PDS system.In contrast with FWW(D-1),only 14.4%DEHP in the FWW(D-90)(on behalf of FWW returned to surface 90 days after hydraulic fracturing)was degraded.The reason was that the high concentration of coexisting halogen ions(Cl^-concentration up to 3577 mM and Br^-concentration up to 15mM)generated hypochlorite with low redox potential,which was difficult to oxidize DEHP directly.(2)In the Fe²⁺/PDS oxidation system,the co-existing suspended particles,PAM and ethylene glycol in fracturing wastewater could synergistically promote the degradation of DEHP.The results showed that the degradation of DEHP was 26.1%in the FWW containing 14-day solids(suspended solids from simulated FWW after 14-day sedimentation)within 30 min treatment by Fe²⁺/PDS.However,the degradation of DEHP was only 12.2%with the addition of Fe²⁺/PDS/14-day solids in ultra-pure water.In Fe²⁺/PDS/14-day solids system,a large amount of DEHP and Fe²⁺were adsorbed on the surface of particulate matter,and it was found that the oxidative degradation of DEHP mainly occurred in the liquid phase.Moreover,the degradation of DEHP was increased to24.3%after PAM addition.The reason was that surface of the 14-day solids could adsorb more PAM to retain more DEHP and Fe²⁺in the solution for oxidative degradation.After the addition of ethylene glycol,the degradation of DEHP was increased to 30.1%.The reason was that there was a positive synergistic effect of PAM and C₂H₄(OH)₂ on DEHP degradation,which was ascribed to the reductive^·C₂H₃(OH)₂ replenishing Fe²⁺for improving Fe²⁺/PDS oxidation.In contrast with 14-day solids system,only 9.31%DEHP in the FWW containing 1-day solids(suspended solids from simulated FWW after 1-day sedimentation)was degraded.The reason was that surface of the 1-day solids had small porosity and pore size which could not adsorb high-molecular PAM effectively to retain DEHP and Fe²⁺in the solution for oxidative degradation.Hence,the DEHP degradation was decreased in 1-day solids system.(3)Fe⁰/PDS was established to degrade DEHP in FWW.And it was found that the coexisting barium ions in wastewater could inhibit DEHP degradation,while ethylene glycol could promote DEHP degradation.The results showed that DEHP degradation in FWW(D-1)and FWW(D-14)(representing the wastewater returned to the surface on the1/14 day after hydraulic fracturing)was 8%?11.0%,which was much lower than the degradation efficiency(25.0%?41.5%)in Fe²⁺/PDS oxidation system.Besides,the degradation of DEHP was 11.1%with the addition of Fe⁰/PDS in ultra-pure water.But the addition of Ba²⁺into the Fe⁰/PDS reaction system significantly inhibited DEHP degradation(DEHP degradation was less than 1%).The reason was that Ba²⁺could react with SO₄^2–to generate a large amount of barium sulfate precipitation,which adhered to the surface of iron powder.The barium sulfate inhibited the contact and activated process between Fe⁰ and PDS,then inhibited the generation of oxidizing free radicals in the solution.However,the degradation of DEHP was increased to 10.2%after the addition of ethylene glycol.The reason was that the reductive^·C₂H₃(OH)₂ promoted the corrosion of zero-valent iron,enhanced the activation process of PDS,and then enhanced the oxidative degradation of DEHP in the Fe⁰/PDS/Ba²⁺system.(4)According to the composition of actual fracturing wastewater,the degradation rate of DEHP could be effectively improved by regulating the coexisting components of wastewater.The composition of actual fracturing wastewater from an oil and gas field in Yichang,Hubei Province,was studied and analyzed.Compared with the simulated wastewater,the actual wastewater contained lesser small molecular organic accelerators such as ethylene glycol and higher concentration of barium ion.The results showed that the degradation rate of DEHP and TOC could be increased to about 2 times and 1.5 times respectively by adding ethylene glycol in Fe/PDS oxidation systems.Moreover,the degradation rate of DEHP and TOC could be increased to more than 3 times and 2 times respectively by sulfuric acid pretreatment and glycol addition in the Fe⁰/PDS oxidation system.The oxidation and degradation of DEHP in actual wastewater could reduce the environmental risk of wastewater,and the agent costs of oxidation systems were lower than 5(?)m^–3.