Competitive Adsorption And Desorption Behavior Of Loess Soil Towards Pb(Ⅱ),Cu(Ⅱ),Cd(Ⅱ)

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 Pb(Ⅱ), Cu(Ⅱ)、 Cd(Ⅱ) on loess soil in single system, binary system and ternary 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)In single system, soil-liquid ratio, solute concentration, reaction time, temperature and pH 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) and Cu(Ⅱ). 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 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) and Cu(Ⅱ) are much better than Cd(Ⅱ). The heavy metal sorption on loess soil follows the sequence of Pb(Ⅱ)>Cu(Ⅱ)> Cd(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. Hydrated ion radii also have great effect on the sorption of heavy metals on loess soil. 3) Solute concentration and soil-liquid ratio have important impact to competitive adsorption. The competition effect between heavy metals can be enlarged by increasing the concentration of heavy metal ions. Lncreasing soil-liquid ratio can lead to higher removal efficiency, but lower unit adsorption amount. And obivious mutual inhibition can be found under acid condition, while the competition effect will decrease with increasing pH under alkaline condition.4) NTA and EDTA can facilitate the release of Pb(Ⅱ), Cu(Ⅱ) and Cd(Ⅱ) from contaminated 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 organic ligands of desorption agent can combine with the heavy metal ions in solution or the loess, which can decrease the adsorption capacity.