Stabilization Of Chromium-contaminated Soil With Surface Modified Activated Carbon

With the rapid development of industry and agriculture, heavy metal contamination in soil has become increasingly serious. Chromium has posed an enormous threaten to the environment especially resulted from the compolicated forms and tremendous toxic and mobile hexavalent chromium. Among most methods for the remediation of heavy metals-contaminated soil, stabilization/solidification has been widely used to stabilize/solidified heavy metals in sediment, sludge, industrial waste and soil due to the advantages of cost-effective, widely application and simple operation. But the key of stabilization/solidifiication is to search for cost-effective stabilizer. Activated carbon with well developed internal pore structures and large specific surface area has been applied to adsorb heavy metals in gas system and liquid system, but few application in solid system.In this dissertation, activated carbon was modified to enhance surface chemical porperties for the remediation of chromium-contaminated soil. Moreover, the mechanism of modified activated carbons stabilizing chromium in soil was discussed.Commerical activated carbon(marked with AC) was modified with HNO3, NH3 and Na2 S to acquire functional groups containing O, N, S(marked with OAC, NAC and SAC)respectively. The physical and chemical properties of activated carbons were characterised by BET, elemental analysis, FT-IR, Raman, XPS, Boehm titration and p Hpzc. The results showed that the specific surface area of OAC was enlarged by 27.0%,oxygen containing functional groups including-COOH and-OH were generated while the content of C=O was decreased during the process of modification; the specific surface area of NAC was increased by 20.2%, and pyridine, pyrrole and pyridine oxide and the like of nitrogen-containing functional groups were introduced; the specific surface area of SAC was decreased by 34.6%, and the sulfur functional groups of S-S,thiophene and O=S=O were acquired by the process of sulfur impregnation.To explore the effect of stabilization of chromium by modified activated carbons,BCR sequential extraction was employed to evaluate the horizontal and vertical distribution of chromium in contaminated soil collected from a chromium salt plant. We found that in the soil samples with 2.41%~39.88% total chromium in exchangeable form,both the total chromium and hexavalent chromium decreased differently after stabilization with 5%(w%) AC, OAC, NAC and SAC, in which the content ofhexavalent chromium can reduce to the lowerst detectable limit. The ability of four ACs to stabilize total chromium is SAC>OAC>AC>NAC.To further study the bioavailability of chromium in soil, the pot experiment was conducted with green pepper as the indicator plant. The measured mass of root, stem,leaf and fruit, and the concentration distribution of chromium implied that OAC was able to surpress the bioavailability and mobility of chromium in soil so as to reduce the toxicity of chromium to pepper. Chromium amount accumulated in the pepper presented the following tendency: root>leaf>stalk>fruit. Besides, in the chromium-contaminated soil of 300mg/kg, more than 90% chromium was riched in the roots.Adsorption isotherms and adsorption kinetics of Cr(VI) and Cr(III) with the above-mentioned four ACs were investigated. Langmuir model was applied to describe the adsorption isotherms of Cr(VI) and Cr(III) for AC, while Freundlich model was applied to describe the adsorption isotherms of Cr(VI) and Cr(III) for OAC, NAC and SAC due to surface inhomogeneity caused by the functional groups acquired during the process of surface modification. Pseudo-second order kinetic model was applicable to described the adsorption kinetics of Cr(VI) and Cr(III) for AC, OAC, NAC and SAC,indicating that adsorption of Cr(VI) and Cr(III) on ACs is a chemical adsorption process.In addition, the equilibrium adsorption capacity of Cr(VI) for OAC, NAC and SAC increased by 112.17%,120.73% and 55.20% at 298 K respectively compared with AC.Combining FT-IR, Raman and XPS to analyse the properties of AC, OAC, NAC and SAC before and after adsorbing Cr(VI) and Cr(III) showed that the functional groups(C=O, C-O-C, C-O or O-S, Pyridine, S-S, Thiophene, O=S=O) introduced by the modification of activated carbon have enhanced the contribution of chemical effect in the adsorption process.