Technological Research Of Adsorption Of Heavy Metals By Modified Activated Carbon

In recently years, the frequent incidentsinvolving heavy metal contamination hadjeopardized local drinkingwatersafety and foodsecurity and, to a larger extent, posed aserious threat to human health in China. The traditional heavy metal removal process issuitable for removing high concentrations of heavy metal ions, while for low concentrationsof removal techniques—such as the reverse osmosis and others—the cost is too high;therefore, it is urgent to develop efficient, convenient heavy metals removal techniqueswith reasonable cost. Activated carbon adsorption is one of the most important watertreatment methods, and has better effects for the removal of organic pollutants, havingadvantages such as materials are readily available, relatively inexpensive, etc. Theadsorption of cadmium, copper, lead, chromium and other heavy metals by activated carbonis minor, but that can be improved by modifying the surface properties of activated carbonto promote the adsorption of heavy metals. Therefore further exploration and research forthe modification methods and mechanismsof activated carbon is theoretically important andpractically useful.This study is to further enhance the effectiveness of activated carbon for the removalof heavy metals. In this study, one type of commercially available activated carbon wasmodified by mixed acid oxidation, ozone oxidation and ethylene-diamine, getting AC-O,AC-O3, AC-NH2.The best method was selected in accordance to the removal rate of Cd, Cu,Pb and Cr (VI) and the adsorption rate. The changes of the properties were examinedthrough the analysis of surface area and pore, IR spectroscopy and XPS and thedetermination of (surface point of zero charge) pHpzc. Then the preliminary study of theadsorption mechanism was conducted. Then the influence factors such as pH and calciumions were analyzed via adsorption thermodynamics. The mechanism of competitiveadsorption was analyzed by means of the Koopal model. Finally the cadmium adsorption inthe pure water and deionized water by the fixed bed was studied.The results show that the modified activated carbon has demonstrated enhancedadsorption effects;Acid-modified activated carbon has the best adsorption capacity for Cd, Cu, Pb, amine-modified activated carbon for Cr (VI), and the surface properties of themodified activated carbon has been greatly changed. The changes in the physical propertiesof the surface have little impact on the absorption, because the surface area decreased afterbeing modified, but the adsorption effect was improved. The order of pHpzcis AC-O <AC<AC-O3<AC-NH2.The pH value determines the species of the metal ions, when the pH isgreater than pHpzc, the carbon surface is negatively charged; also the greater the pH is or thelower theelectrostatic pHpzcis, the more robustlythe positive ions are adsorbed.In the singleion adsorption system,Langmuir and Freundlich models both fit the Cd adsorption, and pHparameters show that the higher pH is, the better the adsorption will be. In the competitiveadsorption system ofCa and Cd, the adsorption capacity of Cd is stronger.In the experiments of adsorption by fixed bed, the initial concentration of the cadmiumwas about60g/L. After17days, the effluent concentration of the solution made in purewater was about1.7g/L, Cadmium adsorbed by AC-O per unitmass: QCd＝18.64mg/g.The effluent concentration of the solution made in natural water wasclose to the limit5g/L after65hours, greatly affecting the efficiency of Cadmium.