| Zero-Valent Iron Permeable Reactive Barrier (Fe0-PRB) is a competitive andeconomical in-situ groundwater remediation technology in recent years, and highperformance of Cr(VI)-polluted groundwater removal by Fe0-PRB has beenrecognized。 However, passivation of Fe0caused by mineral fouling, resulting indiminished removal efficiency has become main problem for long-term performance ofPRB systems. In this study, based on the understanding of removal and passivationprocess of PRB systems, a new electrochemical permeable reactive barrier(Electro-PRB) system was proposed. The process and Mechanism of electrochemicaldepassivation were studied.Research on reaction kinetics and impacting mechanism revealed removal processcan be described by the kinetics expression asd [C r (V I)]/dt=k (A)[Cr (V I)]2/3[H+]2/25Theion (chloridion, bicarbonate, sulfate, calcium, magnesium, calcium and bicarbonate,magnesium and bicarbonate) with different concentration lever may influence Cr(VI)removal through participating in the reaction or impacting electron transfer.The analysis of chemical form and adhesion mechanism of Cr on mineral-solutioninterphase revealed sorption was the key machanism between magnesium carbonate,calcium carbonate, magnesium hydrate, calcium hydrate and Cr(VI), separately. andCr(VI) removal by ferroferric oxide or lepidocrocite was through combined effect ofsorption and reduction. The interation process may impact coordination number andbond length of Cr on mineral-solution interphase.Periodic application of electrochemical depassivation after Fe0passivation bycleaning the passivation layers may restore the Fe0reactivity. It was found that initialremoval rate was restored, and longevity was prolonged55.4-118.3%. Duringelectrolysis process, the Fe ion generated from anode may accelerate Cr(VI) removal inelectrolyte with new minerals formed. Furthermore, no dissociation and oxidation ofCr(III)-precipitates was discorvered, and no secondary pollution in effluents wasdetected. The declined pH and increased ORP were found during passivation process.The results indicated electrochemical depassivation was capable of sustainableoperation of PRB systems.The performance of Electro-PRB systems under various geochemical conditions was examined. Chloridion impacted the passivation process by influencing formation ofpassivation layers, without participating in minerals fouling. However, bicarbonate,sulfate, calcium, magnesium, calcium and bicarbonate, magnesium and bicarbonateparticipated in mineral fouling to impact passivation. However, mineral layers werecleaned by electrochemical depassvation, and PRB longevity prolonged with initialreactivity recorved.The optimized electrolysis control parameters (electrolysis voltage/time,electrolysis distance) were investigated. Removal recovery rate increased as electrolysisvoltage/time raise before meeting the most appropriate electrolysis voltage/time, andthen decreased as electrolysis voltage/time varied higher. The Cr(VI) removal raterecovery decreased significantly with increased electrolysis distance.In this study, system modules of electrochemical depassivation were designed forcaisson installation technology and trench installation technology, respectively. Whichimprove the applicability of Electro-PRB system. |
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