Study On In - Situ Passivation Stability Of Heavy Metal Contaminated Soil

With the increasing of heavy metal contamination of soil, the remediation ofheavy metals contaminated soil increasingly becomes a hot environmental issue. Insitu immobilization remediation is a technology that add passivator to soil, then aseries of reactions like adsorption, precipitation, complexation, ion exchange andredox will happen between passivator and heavy metal, reducing the transferabilityand bioavailability of heavy metals. Because of its low cost, high remediationefficiency, and easy operation, the remediation of contaminated soil with medium-lowconcentration has good application prospects. However, the fatal weakness is that itonly reduces the transferability and bioavailability of heavy metals, the stable heavymetals after passivation still remains in soil, with the change of time and environment,they may become reactivation threats.The paper discusses the stability of in situ immobilization remediation from twoaspects: the stability of passivator and the combined stability between passivator andheavy metals. Two passivators-Na-B and MBC were selected. Through simulationexperiment and incubation experiment, to study the thermal stability, chemicalstability (chemical oxidation stability and pH stability) and biological stability of thepassivator itself. Through the adsorption and desorption experiment, to study thestability of combination between heavy metal and passivator and its influencingfactors. Through pot experiment, to study the stability of passivator in the practicalapplication. Its purpose is to provide a theoretical basis for the passivator actuallyapplied in situ immobilization remediation of heavy metals in contaminated soils.The main results of this study are as follows:MBC was selected as passivator, and found that:(1) The thermal stability ofMBC is well, using thermal gravimetric analysis of the MBC gradually heated to700℃, the quality of MBC reduced13.01%.(2) The chemical stability of MBC iswell. When using333mmo·L-1KMnO4oxided MBC for24hours, the maximumdegradation rate is4.22%. MBC has a quite strong pH stability, a acid-alkali corrosionto MBC with different pH solution. When pH=2, the degradation is1.933%; whenpH=12, the degradation is5.334%.(3) MBC has a good biological stability inconcentrations below105cfu·mL-1of Bacillus subtilis. When the concentratian above105cfu·mL-1, the biological stability becomes worse alone with increasedconcentration. At the concentration of105cfu·mL-1and cultured for30days, the maximum degradation rate of MBC is11%. Biodegradation of MBC is greater thanthe chemical oxidation.The adsorption characteristics were investigated in terms of adsorption kinetics,isotherms of Cu2+and Cd2+on the MBC and Na-B. The adsorption of Cu2+and Cd2+on the MBC and Na-B were fitted with both the Langmuir and the quasi-two kineticequation. Batch experiments showed that the adsorption reaction of Cu2+and Cd2+onthe MBC and Na-B was divided into a rapid and a slow process. The maximaladsorption quantities of Cu2+and Cd2+on the MBC were23809mg·kg-1and24390mg·kg-1, the maximal adsorption quantities of Cu2+and Cd2+on Na-B f were4681mg·kg-1and5356mg·kg-1. The n values from the Freundlich equation were greaterthan1, respectively, and exhibited preferential adsorption. That the MBC had higheradsorption properties for the metal ions. The adsorption properties of Cu2+on theMBC and Na-B were better than that of Cd2+.In different conditions, to desorb two kinds of material adsorbed heavy metalswith0.01mol·L-1CaCl2. The results showed that, temperature and salt ionconcentration has little effect on desorption rate, means that temperature and salt ionconcentration has little effect on the passivation stability. While pH has a great effecton the desorption rate, with the decrease of pH desorption rate increases rapidly. Itshows that the higher soil pH, the more stable passivation; the lower soil pH, the lessstable passivation; so the changes of soil pH will have a great influence on passivatorstability.The two passivators actually applied in the remediation of copper contaminatedsoil. It was found that after the application of MBC, there is no significantly changeon the availability in three soils after40days, indicating strong stability of MBC.After MBC applied into the three soils, the reducing of availability in Cinnamon Soilis the fastest. Within10-20days, the availability in Cinnamon Soil reduce most, andthe reducing almost completed within30days. Within30-40days, the availability inBrown Soil reduce most, and the reducing almost completed within40days. Within10-30days, the availability in Acid Brown Soil increased, and the reducing mainlyhappened in30-40days. After the application of Na-B, there is no significantlychange on the availability in Cinnamon Soil and Brown Soil after40days, indicatingstrong stability of Na-B. In Acid Brown Soil, the content changes of availability hasno order, indicating poor passivation stability. After Na-B applied into the three soils,the reducing of availability in Cinnamon Soil is the fastest. Within10-20days, the availability in Cinnamon Soil reduce most, and the reducing almost completed within30days. Within10-20days, the availability in Brown Soil reduce most, and thereducing almost completed within40days.The growth of ryegrass has promoting effect on the passivation stability, in thesoil with MBC and Na-B, the content of heavy metal availability of the soil plantedryegrass is lower than those of non ryegrass planting soil. The reason may be, in thegrowth process, the adsorption of heavy metals by the roots of the plant, the transferfrom roots to aboveground parts, all these can reduce the available heavy metalcontent in soil.