| With the advancement of industrialization and urbanization,the problem of cadmium pollution is becoming increasingly serious in China.Thus,it is urgent to take safe and effective approaches for water and soil polluted by cadmium.At present,biochar,as a lowcost carbon-based material based on agricultural and forestry wastes,has drew extensive attention and research in the process of remediation of heavy metal pollution owing to its good pore development and a wide range of raw materials.Among them,hydrochar,as a new type of biochar material,has received more and more attention in recent years.Compared with conventional pyrolysis biochar,it has many advantages,such as high yield,no dehydration,no smoke,and relatively low energy consumption.However,the related research on the adsorption of heavy metals Cd as a new adsorption material is still in its infancy.Poor development of hydrochar pores limits its ability to adsorb and repair the environment.The improvement of the specific surface area of hydrochar through biological or chemical methods can improve its adsorption performance.At present,there are few researches on the improvement of hydrochar;and the effect of the improved hydrochar on the content and effectiveness of Cd in soil is not clear.In this study,the microbial aging improvement method and chemical improvement method were used to improve hydrochar.The hydrochars prepared from two different sources(wheat straw and poplar sawdust)at two temperatures(220℃ and 260℃).Three aging times were set:20 d,40 d,and 60 d;the chemical modification treatment set three different nitric acid concentrations(5%,10%,and 15%).The characterization of hydrochar before and after improvement was analyzed by material characterization.The adsorption characteristics of cadmium and its adsorption mechanism by different improved hydrochar were studied through batch adsorption experiments.At the same time,the effect of hydrochar on soil microbial diversity was analyzed.The results showed as follow:Both microbial aging improvement and nitric acid improvement have obvious effects on the surface characteristics of hydrochars,and the pore structure is more developed after the improvement.The Zeta potential,surface polarity,aromaticity,and content of oxygencontaining functional groups of microbial aging-improved hydrotchars showed an increasing trend with increasing aging time,while the C content decreased.Compared with sawdust hydrochars,the wheat straw hydrochars treated with microbial aging is richer in oxygencontaining functional groups and the overall polarity is stronger,while the nitric acid modification treatment is just the opposite.Temperature treatment had no significant effect on the characteristics of straw and sawdust hydrochars.The results of batch adsorption tests showed that the nitric acid-modified hydrochars has higher adsorption performance than the microbial aging treatment,and the straw hydrochars has higher adsorption capacity than sawdust hydrochars.The treatment capacity of straw hydrochars is higher,which is nearly 9 times higher than that of unmodified hydrochars.In the microbial improvement treatment,M60-SHC220 had the best adsorption effect on cadmium,and there was no significant difference between different raw material treatments.The Langmuir model fits well the adsorption process of cadmium on hydrochars,and the adsorption kinetics data conforms to the quasi-second-order kinetic equation.The adsorption of cadmium by hydrochars is mainly through the complexation of functional groups,electrostatic adsorption and ion exchange.Among them,the complexation of oxygen-containing functional groups is the main control mechanism.The hydrochars modified by nitric acid was applied to cadmium-contaminated soil,and it was found that it can change the form of cadmium and effectively reduce the content of available cadmium in the soil.The effect of adding 1%SHC220(1%sawdust 220℃hydrochar)on soil cadmium was the lowest,which decreased by 20.79%compared with the control.The cadmium accumulation in rice plants decreased after hydro chars treatment,and the cadmium accumulation in plants became the root cadmium accumulation>straw cadmium accumulation>grain cadmium accumulation.The accumulated amount of treated cadmium was less after N15-SHC220-2%treatment,which decreased by 47.87%.Moreover,the accumulation of cadmium in rice grains has significant differences with respect to soil total cadmium,soil pH,and root biomass.Among them,total cadmium in soil has a positive correlation with it,and residual cadmium and straw biomass have a negative correlation with it.The effect of hydrochars on the bioavailability of cadmium in soil is related to the adsorption performance of hydrotchars to cadmium and changes in soil-related microorganisms.After the application of the improved hydrochars,the soil microbial diversity generally showed a decreasing trend,and the specific microorganisms also showed certain differences among the treatments.Among them,the number of OUT corresponding to the N15-SHC220-2%treatment was the least,indicating that there were fewer species,and different microbial species also have an effect on the cadmium form in the soil.After microbial aging and nitric acid modification treatment,hydrochars can improve the surface characteristics of hydrochar and improve the adsorption capacity of cadmium.The adsorption and non-modification of cadmium in aqueous solution by M60-SHC220 in microbial treatment has been improved by nearly 5 In the nitric acid modification treatment,the adsorption and non-modification of cadmium in the aqueous solution by N15-WHC260 in the nitric acid modification treatment was increased by nearly 11 times.It can be seen that the improvement of nitric acid has a better adsorption effect,but the improvement of microbial aging is environmentally friendly advantage.Treatment with hydrochars also affected the availability of cadmium in the soil.The research results will provide data-related support for the application of hydrochars in the agricultural and environmental fields,especially for heavy metal pollution remediation. |
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