Study On Remediation Of Cr-pb Polluted Groundwater Using The Fe~0-prb

Nowadays, the remediation technology of groundwater pollution is a hot topic in the field of environment. Permeable reactive barrier (PRB) technology is a new in situ remediation technology. It has the characteristics of less cost, stable operation and long-term efficiency. Moreover big progress has been made recently in remediation mechanism, design and construction of the system and the reactive materials in practical projects. Currently, PRB technology is gradually replacing the Pump-Treat technology which has high running costs. In a word, PRB is the developing direction of groundwater treatment technologies. The Fe0 permeable reactive barrier which is widely used was proved that the approach is characteristic with in situ remediation, good long-term performance, many remediable contaminants and low costs. It has been using in many countries and has been dealing with many aspects of research and development in groundwater pollution. It is an effective technology for the remediation of groundwater contaminated by chlorinated hydrocarbons, chlorinated aromatic hydrocarbons, chlorinated pesticides and toxic metals such as chromium, selenium, uranium, arsenic and technetium.Batch and column experiments were conducted to investigate the effect and major effect factors for Fe⁰-PRB removal Cr-Pb contaminated groundwater and confirm the feasibility and effectiveness of Fe⁰-PRB used in the Cr-Pb polluted groundwater. The results were as follow.â‘´Hexavalent chromium was removed quickly and effectively by zero valent iron, removal mechanism was redox and coprecipitation, products are Fe(OH)₃, Cr(OH)₃ and (Cr_xFe_1-x)(OH)₃; the removal ratio was effected by the pretreatment with acid and nickelaqe; the amount of iron, the initial concentration of hexavalent chromium and initial pH; the removal of hexavalent chromium was accompanied by a sharp increase in pH, and a sharp decrease in Eh; Fe²⁺ could be used as an indicator for complete reduction of hexavalent chromium in the acidic condition. q⑵Pb was removed effectively by zero valent iron, removal mechanism was redox and flocculating settling, products were Pb(0), Pb(OH)₂ and PbO·xH₂O; the removal ratio was effected by the amount of iron and initial pH; the removal of Pb was accompanied by a sharp increase in pH, and a sharp decrease in E_h.⑶Cr-Pb polluted groundwater was removed quickly and effectively by zero valent iron, the removal ratio was effected by the amount of iron and initial pH. The removal ratio of Pb was decreased obviously, and hexavalent chromium removal ratio was almost uninfluenced when the two metals existed simultaneously.â‘·The removal kinetics model of Cr(â…¥) and Pb by iron were studied as single pollutant or co-exsiting system.The results showed that the removal of Cr(â…¥) and Pb could be fit to pseudo-first-order reaction kinetics model in single pollutant or co-exsiting system. More iron,biger rate constant; lower initial pH, biger rate constant.⑸The zero-valent iron for Cr(â…¥) removal ratio as follows: Cl^-> SO₄^2-> HCO₃^-> PO₄^3-, in the presence of four inorganic anions. Because the effect of anion on iron and the effect of green rust, the results demonstrated that Cl^- and SO₄^2- enhanced remediation , however HCO₃^- and PO43- inhibited remediation observably when the 1.0 mmol/L inorganic anion existed. All anions except PO₄(3-) enhanced reaction when the 3.0 mmol/L inorganic anion existed. Because of adsorption , Cr(â…¥) removal inhibited by HCO₃^- and PO₄(3-). The zero-valent iron for Cr (â…¥) removal ratio as follows: C₆H₅O₇^3-> C₂O₄^2-> CH₃COO^-, when three organic anions existed. The results demonstrated that C₆H₅O₇^3- and C₂O₄^2- enhanced remediation, however CH₃COO^- inhibited remediation observably. Because of chelation of C₂O₄^2- and complexation of C₆H₅O₇^3-, they enhanced remediation. In the presence of four inorganic anions exist, the zero-valent iron for Pb(â…¡) removal ratio as follows: PO₄^3->Cl^->HCO₃>SO₄^2-. When three organic anions existed, the zero-valent iron for Pb(â…¡) removal ratio as follows: C₂O₄^2- > C₆H₅O₇^3-> CH₃COO^-, and all anions enhanced reaction. This was not only because of increasing activity of iron, but also because Pb(â…¡) could react wih anions and prduce low Ksp compound.⑹The zero-valent iron for Cr (â…¥) removal ratio as follows: Fe²⁺ > Fe³⁺ > Ca²⁺ > Mg²⁺, when different cations existed. The results demonstrated that Fe²⁺ and Fe³⁺ enhanced remediation. This was because that Fe²⁺ could deoxidize Cr (â…¥). Ca²⁺ and Mg²⁺ could react wih OH^- by formation precipitation, so they inhibited remediation. Moreover inhibition of Mg²⁺ was greater than Ca²⁺. The results also demonstrated that Ca²⁺ and Mg²⁺ played inhibition role in Pb(â…¡) removal.⑺The higher ions concentration was, the stronger enhance role or inhibition, when different ions existed. The removal process was accompanied by a sharp increase in pH, and a sharp decrease in E_h. The removal of Cr(â…¥) and Pb could be fit to pseudo-first-order reaction kinetics model when different ions existed. The rate constant was effected by ions kinds and concentration.â‘»With the compost, ZVI, and ZVI-compost as reaction media respectively, the feasibility and effectiveness of the Cr-Pb polluted groundwater remediation by PRB were studied. The results showed that the using of compost, ZVI, and ZVI-compost as reaction media to remediatiton Cr-Pb polluted groundwater was feasibility. The reactor packed with ZVI-compost had a better performance than the reactor packed with compost or ZVI alone.⑼With the ZVI-compost as reaction media, increasing the amount of compost and ZVI could increase the removal effects; moreover the removal effects were little influenced by the time of composting; adsorption was the primary role of compost removal pollutants, microorganisms as a secondary role; and the removal effects could be increased by adding active carbon.⑽With the ZVI, ZVI-compost and ZVI-compost-active carbon as reaction media respectively, the dissoluble iron concentration in effluent were detected. The results showed that the dissoluble iron concentration in effluent of the column(ZVI-compost-active carbon) was lowest, the dissoluble iron concentration in effluent of the column(ZVI-compost) was lower than the column(ZVI); however total iron in effluent of all column could meet the requirement of standards for drinking water quality. So it indicated that the using of Fe⁰-PRB is the safe method for Cr-Pb removal from groundwater.