Enhanced Stabilization Of Heavy Metals In Solid Phase Substrate With Functionalized Activated Carbon

Heavy metal pollution has posed a serious threat to the ecological environment, foodsecurity and human health due to its diverse sources, nonbiodegradability, long residualtoxicity, accumulation in food chains and impairment to metabolism of organs. Comparedwith other methods for the treatment of heavy metal-containing waste residue, thestabilization/immobilization technique is characteristic of the cost-effective, time-efficient,easy operation and in-situ application. Thus, it has been widely applied to the stabilizationof heavy metal-containing sediments, sludge, industrial waste residue and fly ash, andremediation of heavy metal contaminated soils as well. However, high efficient andcost-effective additives are still expected to look forward. Activated carbon is regarded asan effective adsorbent for heavy metals, whereas the existing research works focusedmainly on the liquid and gas phase system. In literatures, research about application ofactivated carbon to sequestrating heavy metals in solid waste, particularly effect ofinteractions near interfacial regime between multi-phases on stabilization/immobilizationof heavy metals, was barely seen. In this dissertation, based on interactions near interfacialregimes between the multiphase such as heavy metallic ion solution, activated carbon andsolid waste, investigation into mechanism of stabilization/immobilization of heavy metalswas carried out by using the chromium-occurring waste residue(CR) and leadcontaminated soil(Pb-soil) as the tested heavy metal contaminated solid waste.Surface functionalization of commercial activated carbon was accomplished bysurface modification with HNO3, NH3and Na2S, respectively, to acquire the oxygen,nitrogen and sulfur-containing functional groups. The N2adsorption at77K, Boehmtitration, elemental analysis, FTIR and XPS were employed to measure the specificsurface area, pore volume, pore diameter and surface chemistry of activated carbons. Theresults showed that the oxygen-containing functional groups are carboxyl, lactonic andphenolic hydroxyl groups; nitrogen-containing functional groups are major in amine andpyridine-like groups; sulfur-containing functional groups include sulfur ether, thiophene, sulfinyl, and sulfite.Leaching of Cr in CR and Pb in Pb-soil were investigated with the standard operationprocedures of static leaching, dynamic leaching and toxicity characteristic leaching toexplore the effect of activated carbon on stabilization of CR and Pb-soil. The resultsindicated that the type of extraction fluids and pH value play an important role in theleaching, which is related to the hydrated heavy metal ions in liquid phase and speciationof heavy metals in solid phase. A first order exponential decay function was used tosimulate the dynamic leaching of Cr. It was found that the concentration of Cr in dynamicleachate was higher than that of static and toxicity characteristic leaching, suggesting thatdifferent measures should be considered for the toxicity assessment of heavy metal insolid phase. In addition, activated carbons added to CR and Pb-soil can significantlyreduce the concentration of Cr and Pb in leachate, particularly, a maximum reduction inleaching concentration of80%with OAC was observed.In order to study the bioavailability of heavy metals, a pot experiment was conductedwith background soil as control, pepper as indicator plant, OAC and NBC as additives inCr-soil and Pb-soil. The mass of root, stem, leaf and fruit, and the concentrationdistribution of Cr and Pb in the different organs of pepper implied that OAC was able tomore efficiently suppress the mobility of heavy metals from its stabilized waste throughthe surface reactions between surface functional groups and heavy metals compared to thatfrom NBC stabilized waste. Interactions near interfacial regimes between the multiphaseof heavy metallic ion solution, soil, rhizosphere and activated carbon were explored by theanalysis of Pb and Cr in root with XPS technique and speciation in soil with BCRsequential extraction.Speciation of Cr(III), Cr(VI) and Pb(II) on commercial activated carbon(AC), OAC,NAC and SAC were analyzed by the modified Tessier sequential extraction. The resultsshowed that nearly more than70%of the adsorbed heavy metals on the activated carbonswas present in the form of the residue which stands for the most stable fraction, while theextractable fraction on OAC, NAC and SAC was all much lower than that on AC, indicating that the chemisorption between the surface functional groups and heavy metalscan enhance the stability of the adsorbed heavy metals.Adsorption performance of Cr(III), Cr(VI) and Pb(II) on AC, OAC, NAC and SACwere investigated in detailed by adsorption isotherm, kinetics and thermodynamics.Langmuir, Freundlich, Dubinin–Radushkevich, Temkin models and pseudo second orderkinetics were applied to describe the adsorption behavior, confirming the adsorption isassociated with the chemisorption mechanism. By means of acquiring insight intoadsorption-desorption, dissociation-deposition, mobilization and stabilization and redoxwhich take place in such a multiple system, and combined with the information of surfacefunctional groups and the chemical state of heavy metals obtained from XPS analysis, weobviously revealed the stabilization mechanism of heavy metals on activated carbonfollowing the complexation, chemical bonds and lone pair electron/π electron conjugationbetween heavy metallic ions and O, N and S heteroatomic surface functional groups.