| Soil heavy metal pollution is a serious threat to human health and environment safety, and need to be solved urgently. As the most active part of the soil, the type and quantity of clay minerals closely related with soil properties, playing an important role in maintaining soil moisture and nutrient, passivating the toxicity of pesticide and heavy metal,sequestrating soil carbon, maintaining soil structure, and filtrating groundwater and surface water. The adsorption characteristicof clay minerals might be helpful to solve the problem of soil pollution by heavy metals, but the comparisons between different soils, different clay minerals and different heavy metals should be further studied. The adsorption and desorption characteristics of heavy metals(i.e. lead(Pb) and chromium(Cr)) in aqueous solution on different clay minerals(montmorillonite, vermiculite and zeolite) were measured using Batch tests. The effects of initial concentration, adsorption time,adsorbent dosage and pH on the adsorption of Cr and Pb were investigated for these three kinds of clay minerals.The adsorption-desorption mechanisms of Cr and Pb on these three clay minerals were analyzed by several adsorption models.On this basis, the effect of clay minerals(montmorillonite, vermiculite and zeolite) on the adsorption of heavy metals in acidified soil(pH<6.5)which had added alkaline chemicals(sodium hydroxide, calcium oxide and calcium carbonate).The results are as follows:1) Montmorillonite, vermiculite and zeolite had similar adsorption characteristics of Cr(VI) and Pb(II). Adsorption isotherm, Langmuir equation and Freundlich equation of chromium were all well fitted for montmorillonite, vermiculite and zeolite,with the size of chromium and lead adsorption capacity showing as vermiculite >montmorillonite > zeolite. For montmorillonite and zeolite,the adsorption of Cr(VI) reached the adsorption equilibrium in 2 h; for vermiculite, the adsorption of Cr(VI) reached equilibrium within 5 h. The adsorption of Pb(II) on clay minerals is a rapid adsorption process, showing that it can reach equilibrium within 2 h.The desorption ability of these three clay minerals showed as: montmorillonite > zeolite > vermiculite. According to the desorption kinetics test of Cr(VI) and Pb(II), zeolite had more easy and fast desorption, while Cr(VI), Pb(II) on vermiculite was the most difficult to be desorbed. Desorption amount and adsorption amount of Cr(VI) and Pb(II) on clay minerals showed a significant linear relationship.Initial concentration, adsorbent dosage, adsorption time and pH have a certain effect on the adsorption of heavy metals on clay minerals.2) Acidified soils under different treatments had different adsorption ability of Cr(VI) and Pb(II). Adsorption isotherm of Cr(VI) and two equations were all well fitted for montmorillonite, vermiculite and zeolite in acidified soils. When calcium oxide, calcium carbonate, and not add alkaline substances in the soil were added, the maximum adsorption capacity of clay minerals was in order of montmorillonite > vermiculite > zeolite. When NaOH were added, the maximum adsorption capacity of clay minerals was in order of zeolite > montmorillonite > vermiculite.In the case of adding different basic materials, the size of the maximum adsorption amount of Pb(II) is not consistent with the clay minerals. When different alkaline materials were added, the maximum adsorption capacity of Pb(II) for different clay minerals was in order of montmorillonite > vermiculite > zeolite. In sodium hydroxide added soils, the maximum adsorption capacity of clay minerals was in order of montmorillonite < vermiculite < zeolite. In calcium oxide added soils, the maximum adsorption capacity of clay minerals was in order of vermiculite > montmorillonite > zeolite. In calcium carbonate added soils, the maximum adsorption capacity of clay minerals was in the order of zeolite > montmorillonite > vermiculite. The adsorption amount of Cr(VI) and Pb(II) in treated soils increased rapidly at the first, then slowly, and reached adsorption equilibrium within 5-10 h. |
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