Effect Of Biochar On Heavy Metal Sorption And Immobilization By Sediment

With sediment contamination by heavy metals aggravating, the remediation of contaminated sediment has become one of big concerns currently. Contaminants such as Cd and Pb are often accumulated in sediments and could release into water under certain conditions. Therefore, development of effective and secure techniques offering great flexibility in contaminated remediation is highly desirable. In situ remediation technology is an efficient way to immobilize and sorption pollutants, because it has many advantages such as cheap, operability, free of secondary polution. Biochar is a carbonaceous solid residue after thermal treatment of carbon-rich biomass under O2-limited and low-temperature (<700℃) condition. The use of biochars as a low-cost sorbent to immobilize metallic contaminants is an emerging and promising treatment technology, which attributed to their high surface, various active functions, high pH and CEC. In addition, wetland ecological restoration planting technology widely used produce large amounts of plant residues, how to deal the plant residues to prevent them from decomposition and cause secondary pollution is a serious problem. Hence, wetland restoration plants, Thalia dealbata and Vetiveria zizanioides L., were pyrolyzed into biochar (were referred to as TD and VZ) to study the effects of biochar on the sorption and immobilization of heavy metals in sediments. Batch equilibrium experiments, column leaching experiments, germinate experiments and pot experiments were performed in the present research to investigate and compare the effect of TD and VZ on the sorption and immobilization of Cd and Pb from sediments. The primary results are as follows:1, Batch sorption experiments were established to compare and evaluate the adsorption capacity of the two types of biochars in single and binary metal ion aqueous solutions. Both VZ and TD had a good ability to immobilize Cd and Pb. Compared with VZ, TD demonstrated a better ability to immobilize Cd and Pb. The adsorption data correlated well with Langmuir model for both TD and VZ, and the maximum capacities (Qm) of Cd, Pb by TD were193.2,227.3mg g-1, respectively, and those of Cd, Pb by VZ were181.1,216.8mg g-1, respectively. The coexistence of both metals reduced their tendency of sorption for both Cd and Pb, while the two biochars had a greater adsorption capacity for Pb than Cd in binary sorption system. It indicated that Cd was more severely affected by the presence of Pb on TD and VZ. In additional, total Qm of Cd and Pb on TD in the binary sorption system (Qm (Cd+Pb)=233.6mg g-1) was much higher than each of them in the single sorption system (QmCd=193.10mg g"1, Qmpb=227.3mg g-1), indicating that TD could be a better adsorbent for multiple contamination than VZ. The change of pH, SEM and FTIR showed that the removal of Cd and Pb by biochar may be attributed to precipitation and the formation of surface complexes between heavy metals with carboxyl and hydroxyl groups on biochar surface.2、The effects of biochars on adsorption and desorption behavior of Cd and Pb in sediments were investigated by batch equilibrium experiment. Both TD and VZ showed good ability to promote the immobilization of Cd and Pb, and TD performed better; while desorption of pre-adsorbed Cd or Pb had a reverse trend. The adsorption data correlated well with both Langmuir model and Freundich model in single system, and the Qm of Cd, Pb by sediment with adding TD were8.8,13.7mg g-1, which were15.26%,22.12%higher than the control, respectively; and those of Cd, Pb by sediment with adding VZ were8.3,11.9mg g"1which were9.34%,5.89%higher than the control, respectively. In binary system, the Qm of Cd, Pb by sediment with adding TD were6.0,7.2mg g-1, which were11.99%,22.71%higher than the control, respectively; and those of Cd, Pb by sediment with adding VZ were5.6,6.5mg g-1, which were4.31%,9.83%higher than the control, respectively. The coexistence of both metals reduces their tendency of sorption for both Cd and Pb in sediments, and the adsorption of Cd was affected more significantly by the presence of Pb in binary sediment system. The results of column leaching experiment further indicated that the presence of boichars inhibited the release of Cd and Pb from sediments, and sediment added with TD exihibited stronger capacity to retain Cd and Pb than VZ. All data suggest that two biochars from wetland restoration plants are potential in-situ capping material to reduce the release of heavy metals in contaminated sediments, and has the ability of applying to the remediation of sediment.3, The effects of biochar on the germination rate and biomass of cabbage were investigated though germination experiment and pot experiment in this study. The germination experiment showed that cabbage germination rate was greater with the application of TD, but time delayed. The pot experiment indicated that sediment pH increased0.03-0.2with the addition of TD, while the Cd concentration in cabbage was significantly decreased, and cabbage biomass and chlorophyll content increased by35.6%-148.7%,40.7%-105.2%with the application of TD, respectively. All suggest TD was particularly effective in reduce phytoavailability of Cd from sediments and promote the growth of the cabbage, due to their high affinity for heavy metals.