Study Of Rhizosphere Influence To The Stability Of The Modified Nanoscale Black Carbon Passivation Of Heavy Metals

In resent years, soil pollution in China continues to deteriorate. The remediation of heavy metals contaminated soilbecomes a hot environmental issue recently. However, the traditional remediation ofheavy metals contaminated soil methods, such as heat treatment, electric treatment and chemical leaching are expensive. In situ immobilization remediation technique is an economical feasible method. It decreases the toxicity of heavy metals by altering the existent form of heavy metals in the soil through adding passivators, which could react with heavy metals such as precipitation, adsorption and ion exchange. In situ immobilization remediation has broad prospects in application. The heavy metals contaminated soil remediation is a technique which fixates the heavy metals into soil by altering their existence. Therefore, heavy metals could be released into soil again, with the variation of external environment resulted by the growing plants. In this paper, we select modified nano-carbon black as heavy metal passivators and study the passivation stability effects of passivators to Cd under the condition of Cd Hyper accumulators Lolium perenne L. and vegetables Cd-tolerant plants Beta vulgaris var. cicla L. Moreover, through water culture experiments, we select the major factors which affect the rhizosphere environment of plants growth, pH and low-molecular-weight acid, to simulate the adsorption and desorption characteristics of modified nano black carbon to Cd²⁺, through which we study the passivation stability effects of modified nano black carbon to Cd. Its purpose is to provide a theoretical basis for the passivator actually applied in situ immobilization remediation of heavy metals in contaminated soils.The main results of this study are as follows: the modified nano black carbon have a good passivation effect on heavy metals Cd under the condition of plant growth. After the application of modified nano black carbon, Cd contents of ryegrass in ground and subterranean part are reduced by 54.9%, 48.5%, respectively; Cd contents of Beta vulgaris in ground and subterranean part are reduced by 54.7%, 39.4%, respectively; the dry weights of ryegrass in ground and subterranean part are increased by 66.0%, 63.0%.In added MBC and not added MBC processing, ryegrass rhizosphere availability concentration Cd than the rhizosphere was reduced by 6.9% and 8.9% respectively; On MBC processing rhizosphere and non rhizosphere soil availability concentration than added MBC respectively treatment decreased by 12.9%, 14.7%, ryegrass rhizosphere and modified nano carbon black in passivation competitive antagonist effect of Cd in soil. In added MBC and not added MBC processing, leaves with Beta vulgaris var. cicla L roots international availability concentration Cd than the rhizosphere was reduced by 6.2% and 0.66% respectively; On MBC processing rhizosphere and non rhizosphere soil availability concentration Cd than applying MBC respectively treatment decreased by 12.3%, 7.2%, leaves with red Tian edible orange roots international with the modified nm carbon black has a synergistic effect when passivation Cd in the soil. The plant growth have impacts on the pH of soil rhizosphere. The pH of rhizophere in Cd contaminated soil is lower than that of non-rhizophere. MBC can alter the heavy metal speciation. When there is no MBC, ryegrass transfer the exchangeable Cd into residual. However, when there exists MBC, the ryegrass transfers the carbonate binding into residual. The Beta vulgaris can transfer the exchangeable Cd into other speciation when there exists MBC.Cd can inhibit the growth of the ryegrass in the water culture condition. The dry weights of ryegrass in the ground and subterranean part decrease by 21.5%, 13.7%, respectively with the stress of 5mg/L Cd. The dry weights of ryegrass in the ground and subterranean part decrease by 42.1%, 31.6%, respectively with the stress of 20mg/L Cd. 5mg/L Cd has no significant inhibit effect on the growth of Beta vulgaris. 20mg/L Cd has significant inhibit effect on the growth of Beta vulgaris, which can result in their death. In the stress of Cd, the cells arrangement and speciation of root hair of ryegrass and Beta vulgaris have no significant variation. pH of these two plants increases rapidly with the stress of Cd. The increase is about one unit and tends to stability eventually.The organic acids secreted by ryegrass are mainly oxalates, and it secretes few tartaric acid 0.15g/L only in the stress of high concentration of Cd. The organic acids secreted by ryegrass increase with the increase of Cd concentration. The production of organic acids decreases with time in the stress of Cd with same concentration. The maximum production of organic acids of ryegrass is 3.41g/L. The organic acids of Beta vulgaris mainly contains oxalates, tartaric acid and malic acid, where oxalates is the majority. The production of organic acids decrease dramatically with the stress of Cd and the production has risen with the concentration of nutrient Cd. The maximum total production of organic acids is 6.94g/L with the stress of Cd. The maximum total production of oxalates, tartaric acid and malic acid are 6.65g/L, 3.20g/L and 1.68g/L, respectively.The absorption-desorption experiment of Cd²⁺ in the modified nano-scale black carbon?MBC? under different pH investigated the effects of pH on absorption-desorption characteristics and stability in this paper. Batch experiments showed that the adsorption isotherms of Cd²⁺ on the MBC were fitted with both the Langmuir and Freundlich equations. The n values were both greater than 1 which exhibited preferential adsorption. The MBC adsorption capacity of Cd²⁺wasincreasedwith the increase of the pH and the maximum adsorption amount was stable when the pH was greater than 9. Instead, The MBC desorption capacity of Cd²⁺wasdecreased with the increase of the pH. When the pH was greater than 9, with the increase of pH, the desorption speed became slow. Finally, it attained a stable status. When the pH was lower than 9, pH had a significant effects on absorption-desorption characteristics and stability of Cd²⁺ on the MBC, and the larger the pH, the better the absorption effect. It can be concluded that the MBC was efficiency on restoring alkaline soil polluted by heavy metal.All the three organic acids can decrease the adsorption of MBC to Cd²⁺, and the descent scopes are malic acid>tartaric acid >oxalates. In different organic acids, Cd²⁺ Batch experiments showed that the adsorption isotherms of Cd²⁺ on the MBC were fitted with both the Langmuir and Freundlich equations. Not to add organic acid and under the effect of oxalic acid, tartaric acid, malic acid, the maximum absorptions of MBC to Cd²⁺ are 28146.9 mg/kg?28523.3 mg/kg?29894.4 mg/kg?25346.1mg/kg, respectively. Oxalates and tartaric acid can increase the maximum absorption of MBC to Cd, and malic acid is on the contrary.All the three organic acids can promote desorption of Cd²⁺ in MBC.The desorption rate in the low concentration increases rapidly. With the increase of the concentration of organic acids, the desorption rate of Cd²⁺ continues to increase. However, the increase rate becomes slow gradually. The abilities of these three acids in desorption to Cd²⁺ are malic acid >tartaric acid> oxalates. The difference between tartaric acid and malic acid is relatively less and the desorption abilities to Cd²⁺ of these two acids are far greater than that of oxalates.