Study Of Combined Biochar And Compo St Amendment On Soil Heavy Metals In Dongting Lake Wetland

Dongting Lake wetland is a kind of important ecosystem and habitats to human productive lives.However,the wetland is gradually decreasing and contaminated seriously in recent years due to the strong interference of anthropogenic activities,causing the potential threat to t he balance of the ecosystem and human health.The heavy metal adsorbed into the sediments by water particles is easy to be released from sediments due to the change of hydrodynamic and physicochemical condit ions,thus causing secondary contamination and ecological environment deterioration.In addition,the metals are difficult to degrade or remove and readily accumulate in soil.Therefore,there has been extensive concern about heavy metal pollution in wetland and it is urgent and essential to wetland restoration and ecosystem restoration.The combination of compost and biochar has been proven to be a very effective method for soil polluted by heavy metals.However,these studies are conducted at certain ratio of biochar and compost,and very few works are conducted to investigate the impacts of various ratios bet ween biochar and compost on contaminated soil.Therefore,in this study we investigated the effects of biochar-compost combinations with different ratios on the mobilit y and availabilit y of metals in Dongting Lake wetland and aimed to?1?evaluate the chan ges in the physicochemical properties of wetland soil amended with variable biocha r-compost mixtures after 60d of incubation;?2?examine the effect of combined biochar-compost on the availability of cadmium?Cd?,zinc?Zn?and copper?Cu?in contaminated wetland soil by CaCl₂-extraction procedure and elucidate the underlying controlling processes mechanistically;and?3?assess the efficacy of biochar-compost combinations with various ratios as immobilizing agent to reduce the mobility of these elements in soil by sequential extraction procedure;?4?compare and analyze the sorption-desorption behaviors of Cd,Cu,and Zn after amendment mixtures applied;?5?determine the sorptiondesorption mechanisms of various ratios between amendments to Cd,Cu,and Zn.In this study,we examined the soil physic-chemical properties,heavy metal availability and speciation of the soil remedied with biochar-compost combinations with different ratios after the incubation experiment.The results showed that applying all amendment combinations into wetland soil increased gradually the total organic carbon?TOC?,water-extract organic carbon?WEOC?,catio n exchange capacit y?CEC?and available phosphorus as the compost percentage rose in biochar-composts.The higher pH was obtained in a certain biochar addition?20%and 40%?in combinations due to efficient interaction of biochar wit h compost.All amendments could significantly decrease availability of Cd and Zn mainly from pH change,and the lower availabilit y was obtained in higher pH,but increase available Cu concentration as the result of increased water-extract organic carbon?WEOC?and high total Cu content in compost.Moreover,amendments can decrease easily exchangeable fraction significantly,and the easily exchangeable fraction of Cd and Zn decreased gradually with increase of compost content,while amendments containing compost promote transformation of Cu from Fe/Mn oxide and residual fractions to organic bindings.These results demonstrate that different ratios of biochar and compost have a significant effect on availability and speciation of heavy metals in multi-metal-contaminated wetland soil.The effects of various ratios between biochar and compost on sorption-desorption behaviors of Cd,Cu and Zn were investaged in this study.The adsorption data of Cd and Zn were well fitted by the Langmuir and Freundlich equation.But for Cu,the Langmuir model could describe the adsorption characteristics better than Freundlich model.The incorporation of amendment combinations into soil significantly promotes the sorption affinity of soil on metals.The sorption capacity of Cd and Zn was improved as the compost percentage rose in biochar-compost more likely due to the increase of organic matter and available phosphorus,while t he sorption capacit y of Cu was higher with 10%and 20%biochar addition in biochar-compost combinations likely as the result of the formation of new specific adsorption sites and the mobile Cu and WEOC adsorption in compost after adding a certain amount of biochar in amendment mixtures.Addit ionally,a certain proportion of biochar applied into amendment mixtures could suppress desorption of Cd and Cu by pH change,and the Zn desorption rate gradually decreased as the compost ratio increased in amendment mixtures.