Sorption And Desorption Mechanism Of Loess Soil Towards Typical Heavy Metals

The rapid development of modern industry has brought many negative effects to living environment in recent years, and frequent incidents of heavy metal contamination has aroused broad interest of people around the world. Based on the pollution assessment and remediation of contaminated soil and groundwater, a series of laboratory experiments have been conducted to study the sorption behavior and mechanism of typical heavy metals on loess soil in single system and binary system, in addition, desorption behavior of heavy metals from contaminated loess soil by means of two comlexing agents (i.e., nitrilotriacetic acid trisodium salt monohydrate (NTA) and ethylenediamine tetraacetic acid disodium salt (EDTA)) has been studied. The main fruits are listed in the following:1) Soil-solid ratio, reaction time, temperature and solute concentration have significant effect on the sorption of heavy metals on loess soil. The sorption efficiency of heavy metals will be increased by increasing soil-solid ratio, and loess soil has shown best affinity for Pb(Ⅱ), Cu(Ⅱ) and Cr(Ⅲ), complete removal of these three ions needing less loess soil dosage than other metals. The kinetics of heavy metals on loess soil are consistent with pseudo-second order kinetics, and the variation of sorption with reaction time has been found to follow two stages, i.e., first stage with rapid increasing sorption amount and second stage with slow increasing sorption amount till reaching equilibrium. Analysis of X-ray diffraction spectrum and Fourier infra-red spectrum of loess soil before and after loading with heavy metals reveal that various minerals and organic matters contribute a lot to the sorption of heavy metals.2) The pH value of solution is a critical factor for heavy metal sorption, and the sorption amount will increase by increasing the solution pH. Nearly complete removal of Cu(Ⅱ), Zn(Ⅱ), Pb(Ⅱ) and Cr(Ⅲ) can be achieved at pH<6, while, complete removal of Cd(II), Ni(II) and Mn(Ⅱ) only occurring under strong alkaline condition owing to part of precipitation.3) The mutual competition and antagonism effect of heavy metals in binary system results in the irregularity of sorption isotherm curves, most cureves not following the four sorption isotherm types (i.e., H, L, S and C shaped curves). The sorption capacities of Pb(Ⅱ), Cu(Ⅱ) and Cr(Ⅲ) are best of all, followed by Zn(Ⅱ) and Cd(Ⅱ), finally Ni(Ⅱ) and Mn(Ⅱ). The heavy metal sorption on loess soil follows the sequence of Cr(Ⅲ)> Pb(Ⅱ)> Cu(Ⅱ)> Zn(Ⅱ)> Cd(Ⅱ)> Ni(Ⅱ)> Mn(Ⅱ), which determined mainly by the mineral constituents and organic matters in loess and properties of heavy metals. The minerals (i.e., quartz, mica, vermiculite, goethite and so on) have played important roles in sorption process. Heavy metals can hydrolyze under alkaline natural pH conditions, so hydrolysis constants and hydrated ion radii also have great effect on the sorption of heavy metals on loess soil.4) The competition effect between heavy metals can be enlarged by increasing the concentration of heavy metal ions, and obivious mutual inhibition can be found under acid condition, while the competition effect will decrease with increasing pH under alkaline condition. Comparing with individual sorption of heavy metals, the pH edges of Pb(Ⅱ) and Cu(Ⅱ) show right shift of different extent in binary system, the sorption of Zn(Ⅱ) showing distinct pH edge different from that in single system. The pH edge of Cr(Ⅲ) sorption in binary system indicates that Cr(Ⅲ) has the best competition sorption capacity. Cd(II) has the worst competition sorption capacity because of formantion of unstable nonspecific out-sphere complexes on loess soil.5) NTA and EDTA can facilitate the release of Pb(Ⅱ), Cu(Ⅱ), Cd(Ⅱ) and Zn(Ⅱ) from contaminated loess soil, in reverse, both has no effect on removing Cr(Ⅲ) from loess soil but help resorption of Cr(Ⅲ) on loess soil. The desorption efficiencies of heavy metals using EDTA has been found better than that using NTA because that EDTA-metal complexes are usually stable than NTA-metal complexes. The different numbers of coordination bonds lead to formation of ligand-like complexes and metal-like complexes respectively when organic ligands react with divalent and trivalent havy metas, and these two complexes has different properties which result in the release of divalent ions and resorption of trivalent ions.