Remediation Of Contaminated Sediments By Heavy Metal Fixation With Biochar

With the development of urbanization and industrialization, a large number of heavy metal pollutants have been input into the water environment by atmospheric precipitation, water and soil erosion, wastewater discharge, rain water leaching, finally enriched in the lake sediments, resulting in an increasing pollution caused by the heavy metals. As a novel environment functional material, biochar has the potential ability for remediation of soil polluted by heavy metals. Biochar could reduce the bioavailability of heavy metals by electrostatic attraction, ion exchange, surface complexation and coprecipitation. However, the research of biochar used in the heavy metal remediation of lake sediments is lack of understanding. The typical heavy metal pollution lake in Daye was sampled to evaluate the current heavy metal pollution of the sediments and the adsorption behavior of biochar, and the fixation of heavy metal in sediment by straw biochar was simulated, to discuss the change of dissolved metals in aqueous phase and the morphology change characteristics of heavy metals with biochar as the amendment. The plant toxicity experiment and PBET（Physiologically Based Extraction Test） were carried out as well. The main results were as follows:（1） Daye Lake sediments pollutted by heavy metals were investigated and evaluated, and the soil background values in Hubei province were set as the evaluation standard. The geological accumulation index method was used to evaluate the pollution status, and the pollution of the four heavy metalsfollowed the sequences: Cd > Cu > Pb > Zn. The most serious pollutant was Cd with extreme strong pollution; followed by Cu, which was mainly strong pollution. The last two metals were the Pb and Zn with moderate pollution and slight pollution respectively. Evaluation results of potential ecological risk index method showed that the risk of the four heavy metal was sorting for Cd > Cu > Pb > Zn. The highest risk of pollutants was Cd, reaching extreme strong risk, and Cu mainly had a strong and moderate risks. The risk of Pb and Zn was slight. All sampling points of comprehensive risk level were extreme strong.（2） The absorption of Cu²⁺、Zn²⁺、Pb²⁺ and Cd²⁺ with straw biochar showed that the absorption capacity of biochar to Pb²⁺ was enhanced gradually with the increase of pH value, and it reached equilibrium when pH value was 5. Besides, the adsorption of heavy metals by biochar could be divided into two stages, which was the rapid absorption stage and the slow absorption stage. Meanwhile, the absorption process of biochar to the four kinds of heavy metal was all consistent with the second-order kinetic equation. By comparing the different adsorption rate and equilibrium time of the four types of heavy metal, it could be found that the absorption rate of straw biochar to Zn²⁺ was the fastest, and the absorption rate to Pb²⁺ was the slowest. However, when it reached the absorption capacity, the highest was the absorption capacity of straw biochar to Pb²⁺, which followed by Cd²⁺、Zn²⁺ and Cu²⁺. Hence, the straw biochar could be used as adsorbent and applied to the remediation of water heavy metal pollution.（3） Remediation of contaminated sediments by metal fixation with different addition biochar showed that the biochar could improve the pH value of sediment, while with the increase of addition, the sediment Eh value would also has a tendency to raise. On the other hand, With the increment of biochar addition,the concentration of dissolved metals in overlying water reduced significantly（P<0.05）. Compared with the control group, when biochar application rate was 5%, the concentration of dissolved Cu and Zn in pore water reduced significantly with the increment of biochar（P<0.05）, which reduced the risk of pore water to the overlying water. The experiment also showed that the immobilization of biochar significantly reduce the concentration of acid soluble state of heavy metals in sediments, and the acid-soluble reduced gradually with the increase of time. In the experiment, when biochar application rate was 5%, the decreased magnitude reached 79.4%, and the acid soluble state present a downward trend with time. However, the oxidation and residue fraction increased significantly in this experiment.（4） Biochar had no significant influence on the germination rate of Lepidium sativum, but it promoted the growth of Lepidium sativum significantly. When biochar application rate was 5%, the length of stem and root for Lepidium sativum increased by 25.7% and 32.6% respectively. As the experiment of PBET proves, the PBET Cu、Zn、Pb and Cd reduced significantly with the increase of biochar, especially for the decrease of PBET Cu with 9.8%. In addition, the leached concentration of heavy metal from sediments decreased significantly. Overall, the addition of biochar reduced the toxicity and bioavailability of heavy metals in the lake sediments.