Experimental Study On The Remediation Of Chromium Contaminated Groundwater With PRB Media

So far, there are75chromate enterprises in China and over61enterprises have been shut down due to their small scale, backward techniques and high environmental pollution. Not only the soil around the closed chromate enterprises had been severely contaminated, but also there were serious environmental pollutions in the groundwater. For example, Haibei Chemical Plant in Qinghai Province, the Cr(Ⅵ) concentration of surrounding groundwater was up to42mg·L-1; Qingdao Red Star Chemical Factory in Shandong Province, the Cr(Ⅵ) concentration of surrounding groundwater was about10mg·L-1. As the progress of chromium contaminated soil remediation all over around China, the treatment of Cr(Ⅵ) contaminated groundwater is a matter of great urgency.This manuscript was a part of863subject-Soil remediation equipment research and development and demonstration of Chromium slag contaminated site. This topic research object (in-situ zero-valent iron-Permeable Reactive Barrier, Fe0-PRB) was determined by remediation technologies preliminary screening matrix and remediation technologies detail screening matrix.The reduction of Cr(Ⅵ) by Fe0is self-inhibiting and Fe0easily agglomerates in groundwater. The Cr(Ⅵ) removal mechanism in this system is supposed to involve reduction of Cr(Ⅵ) to Cr(Ⅲ), coupled with the oxidation of Fe(0) to Fe(Ⅱ) or Fe(Ⅲ) and the subsequent precipitation of insoluble Fe(Ⅲ)-Cr(Ⅲ)(oxy)hydroxides. The insulating precipitate inhibits electron transferring from Fe0to Cr(Ⅵ), thus stopping the redox process and causing the Cr(Ⅵ) removal rate to decrease with increasing Cr(Ⅵ) concentration and reaction time. To overcome these deficiency, the study adopted sodium alginate (SA) immobilizing cast iron (SAC) and copper-iron bimetallic particles (SAB) respectively.Experimental results of SAC demonstrate that SA was the best immobilized material. According to analysis with FEI and EDX, pore structures were created by cross-linking of SA with Ca2+, in which a lot of attaching points exist, and through which Cr(Ⅵ) could react with interior iron powder. SA immobilized cast iron (SAC) and reduced iron (SAR) were tested to treat chromium-polluted groundwater individually; the results showed that the removal efficiency of Cr(Ⅵ) by SAC was double that by SAR. After optimization of technology parameters of SAC, the Cr(VI) removal process follows the pseudo first-order kinetics. Based on dynamic experiments with SAC, Cr(Ⅵ)/Fe0is up to32.25mg.g-1and the PRB still maintains high permeability coefficient (2.38cms’1) after complete reaction.Experimental results of SAB demonstrate that application of SA and copper-iron bimetallic particles dramatically increase the removal capacity of Cr(VI) by Fe0. SA and copper-iron bimetallic particles can achieve the maximum efficiency when copper loading of SAB is0.9%, and contribution rate of SA and copper-iron bimetalfic improve the utilization rate of Fe0achieved balance (55%). Cr(VI)/Fe0of SAB0.9is up to40.3mg.g-1. As indicated by Field emission scanning electron microscope (FESEM) and X-ray photoelectron spectroscopy (XPS), copper loading on the surface of Fe0(present a loose layer structure) and acted as catalyst to improve the utilization rate of Fe0through variation of valence to transfer electrons. Designing thickness of PRB through dynamic experiments of SAB0.9, processed the design capacity, effluent Cr(VI) concentration was still less than0.05mg·L1and resolve the deficiency that Fe0-PRB is prone to compaction and blockage.Compared with cast iron mixed with sand, SAB0.9has the distinct advantage of price, and the media cost can save80%. Using SAB0.9as the PRB media is feasible in the remediation of Cr(VI) contaminated groundwater.