Treatment Of Soil Washing Effluent By UV-based Advanced Oxidation Processes

Soil contamination by toxic and hazardous pollutants,especially by the recalcitrant hydrophobic organic compounds(HOCs)is a matter of significant public,scientific and regulatory concerns worldwide.Polycyclic aromatic hydrocarbons(PAHs)are typical representatives of HOCs and they are frequently and readily detected in the contaminated soil due to their ubiquitous occurrence,long-term persistence and low solubility in water.Soil pollution by PAHs may occur singly or co-occur with heavy metals.Ex-situ soil washing has been proposed as a promising remediation technology for elimination of PAHs and heavy metals from contaminated soils due to its high extraction efficiency and low remediation cost.Nevertheless,the soil washing process generates a large amount of effluent containing soil pollutants and additives,for example surfactants and chelating agents,which should be further treated with appropriate methods for the complete elimination of soil pollutants.Ultraviolet(UV)based advanced oxidation processes(AOPs)(UV-AOPs)are promising treatment technologies with respect to efficacy,environmental and economic considerations,and UV-AOPs have been extensively investigated for the oxidation of various hydrophilic organic compounds in the area of wastewater treatment.However,the investigation on treatment of PAHs containing soil washing effluent by UV-AOPs is pretty limited.In particular,research on the recovery of washing agent from co-contaminated soil washing effluent by UV-AOPs after remediation of heavy metals and PAHs co-contaminated soil by soil washing has not been reported.In this dissertation,phenanthrene(PHE)was used as the model of PAH,the feasibility of a variety of UV-AOPs for treatment of PHE contained soil washing effluent was investigated,and then expanded to recover washing agent from co-polluted soil washing effluent by UV and electrolysis coupled persulfate(PS)system after restoration of Cu and PHE co-contaminated soil by soil washing processes.The main content and results consist of:(1)The degradation of PHE in mimic and real soil washing effluents using UVB light activation of hydrogen peroxide(H₂O₂)and peroxodisulfate(PDS)oxidation processes was compared.The results revealed the PHE oxidation efficiency followed the order of UVB/PDS>UVB/H₂O₂>UVB.PHE decomposition was greatly impacted by oxidant concentration while the initial solution p H has little influence on PHE decomposition.The effect of inorganic anions(Cl^-,HCO₃^-and NO₃^-)on the PHE degradation efficiency was evaluated under different p H conditions.Chloride was found to play different roles in the two activation systems.The influence of bicarbonate in UVB/PDS for PHE elimination was negligible,while an inhibition effect was observed for UVB/H₂O₂ system.Nitrate inhibited the PHE degradation in both UVB/H₂O₂ and UVB/PDS processes.Application of the two activation processes on three real soil washing effluents indicated that PHE oxidation efficiencies in the PDS processes were higher than with H₂O₂,and up to 85.0%of PHE degradation could be reached under 6h UVB irradiation with PDS.Finally,energy consumption in the two AOPs was compared and UVB/PDS was suggested to be more cost-effective process than UVB/H₂O₂.(2)Iron citric acid(Fe(III)-CA)was employed in the photo-Fenton processes to remove PHE from mimic soil washing solution.The introduction of Fe(III)-CA strongly intensified PHE oxidation when compared with photo-Fenton process using traditional iron ions,and Fe(III)-CA was more efficient for enhancement of PHE oxidation than other organic ligand chelated Fe(III)complexes.It was mainly due to the rapid photolysis of Fe(III)-CA to produce Fe(II),and improved the Fe(III)/Fe(II)cycle and the decomposition of H₂O₂ to yield active species with the present of H₂O₂.PHE decay obey a two-stage profile and the oxidative degradation rate of the two stages and total removal efficiencies of PHE were mainly impacted by Fe(III)/CA ratio,Fe(III)-CA dosage,H₂O₂ concentration,and initial p H.According to the radical scavenging tests,HO^·was the dominant active species for PHE oxidation though O₂^·-were also involved in the photo-Fenton process.(3)The oxidation of PHE in mimic soil washing effluent containing Tween 80(TW80)was performed by enhanced activation of PS using iron ethylenediamine-N,N-disuccinic acid(Fe(III)-EDDS)complexes under simulated solar light irradiation.The presence of Fe(III)-EDDS enhanced PHE oxidation in a wide p H range of 3.0-8.5,and PHE kinetic presented two different decay steps.The rise of PS concentration increased the oxidation efficiency of PHE.With 30 m M PS and 0.5 m M Fe(III)-EDDS,the PHE removal reached as high as 98.8%after 150 min irradiation.Since the second order reaction rate constants of TW80 and PHE between SO₄^·-are similar,TW80 has an inhibitory effect on PHE degradation,and higher TW80 dosage exhibited stronger inhibition of PHE degradation.The effect of Cl^-on the degradation of PHE was investigated,and it was found that the presence of Cl^-significantly promoted the degradation of PHE,which was explained by the higher selectivity of the generated radicals toward PHE.The oxidation mechanism of PHE was explored through quenching experiments,and SO₄^·-was indicated as the main active species for PHE degradation.(4)After remediation of Cu-PHE co-contaminated soil by soil washing processes,the recovery of extracting agent from soil washing effluent by photoelectrochemical PS system(PE/PS)was explored.To abtain co-polluted soil washing effluent by Cu and PHE,the desorption efficiencies of Cu and PHE from soil using different combinations of surfactants and chelating agents were compared,results showed that the reagent of the combination of TW80 and EDTA exhibited best performance for simultaneous removal Cu and PHE.With TW80 concentration of 10 g L^-1,EDTA dose of 1 m M,and L/S of 20 m L g^-1,the desorption efficienies of PHE and Cu were 68.1%and 73.5%,respectively.The soil washing effluent was then treated by different systems,the results indicated that the PE/PS system could synchronously remove PHE and Cu from the effluent,and electrolysis was mainly responsible for the removal of Cu,while the degradation of PHE mainly depended on the active species produced by photoactivated PS.With PS concentration of 10 m M,current density of 4 m A cm^-2,and irradiation for150 min,PHE was almost completely degraded,the deposition of Cu reached 83.6%,while the recovery of TW80 was 94.0%and of EDTA was 36.8%.Based on the X-ray photoelectron spectroscopy(XPS)analysis of the cathode,electron paramagnetic resonance(EPR)analysis and radical quenching experiments,it was believed that the Cu in the eluent was reduced to Cu⁰ and deposited on the cathode,while SO₄^·-was main active speies responsible for PHE degradation.The recovered agent could be recycled for further 3 washing cycles and maintained a high extracting capacity of PHE and Cu from soil after replenishment of the loss to the initial concentration.