Enhanced Adsorption Of Heavy Metals In Aqueous Solution With Multifunctionalized Biochar

Biochar is a carbon-rich product derived by pyrolysis of biomass in absence ofair. Compared with common activated carbon, biochar naturally contains hetero-atomicfunctional groups such as N, O, S and P which are supposed to be active sites for fourheavy metal ion adsorption. Its internal abundant capillary structure is advantageous toform mesopores and macropores and improving the performance of adsorption kinetics.In this paper, we functionalized biochar in four ways: post oxidation with HNO3,NH3treatment at high temperature, soaking treatment with Br2and Na2S. Additionally,the property changes ofbiochars before and after functionalization were explored andthe performance and stability of adsorption of Pb (II), Cd (II), and As (V) onto biocharwere studied. The results were obtained as follows:Experimental studies have shown that surface functional groups on biochar can bealtered by functionalization treatment which greatly enhances adsorption of heavymetals. Compared to those before functionalization, ammonation by NH3treatment ishelpful to increase specific surface area and pore volume of biochar, while it looked asif the other three methods did not work so. Post oxidation with HNO3desplayed theincreased carboxyl groups and decreased carbonyl groups. Sulfur and bromineimpregnation on biochar led to an increase of carboxyl groups and a reduction oflactonic and carbonyl groups. Finally, ammonation by NH3treatment could reducecarboxyl but increase carbonyl.Adsorption experimental results displayed that favorable pH value for adsorptionof Pb(II), Cd(II) and As(V) was6,6and7respectively. The adsorption seemed tochange little after6hours indicating attainment of equilibrium. the obtained adsorptiondata of Pb(II), Cd(II), As(V) could be fitted well with isothermal adsorption models. In addition, the adsorption kinetic data was found to fit well with a pseudo-second-orderrate equation. And for all the three kinds of heavy metal ions ammonia treated biocharshowed the best performance of adsorption. It were observed that Pb(II), Cd(II) andAs(V) were adsorbed on active sites associated with oxygen-containing andnitrogen-containing functional groups through chelation, which were indicated by FTIR.In a binary aqueous system of Pb(II)-Cd(II), adsorption of lead will depressadsorption of cadmium on biochar, resulting in the adsorption capacity of lead is greaterthan that of cadmium. In the As(V)-Cd(II) system, co-occurrence of cadmium andarsenic can promote adsorption each other, resulting the adsorption capacity of arsenicis much greater than that of cadmium though so does the cadmium; Similarly in theAs(V)-Pb(II) system, co-occurrence of the lead and arsenic can promote adsorption eachother while the adsorption capacity of arsenic is much greater than that of lead thoughso does lead.Stability of the adsorbed heavy metallic ions of Pb(II)、Cd(II)、As(V) on biocharswas investigated by BCR stepextraction. The adsorbed heavy metals on biochars beforeand after functionalization with methods of ammonation by NH3treatment, postoxidation with HNO3, impregnation of sulfur and bromine by Na2S and Br2treatment ismajor in form of residual which is the most stabilized speciation, indicating anenhanced reduction of mobility of heavy metals in ecosystem.