Effect Of Hydrochar On The Phytoavailability Of Pb(Ⅱ) In Heavy Metal Contaminated Soil

The rate of soil lead pollution exceeds the standard spot in China is 1.5%,and it is significant to carry out soil lead pollution treatment to ensure food security and improve soil-water environmental quality.Hydrochar（HC）with a large number of oxygen-containing functional groups on the surface has good performance in adsorption of heavy metals,and combined with the porous structural characteristics of HC material itself,it has been used for heavy metal passivation in contaminated soil in recent years.It was found that the structure of HC would change and exfoliate a large number of oxidized debris（hydrochar-debris,HD）driven by alkalinity,which would have an impact on the environmental mobility of the adsorbed state Pb（Ⅱ）on the surface of HC,and thus induce changes in its phytoeffectiveness,but the current mechanism study related to this needs further improvement.In this study,we used Na OH to treat HC and isolate bw HC（base-washed Hydrochar）HD,and used spectroscopic characterization,adsorption experiments and environmental simulation experiments to study the physicochemical properties,adsorption capacity and adsorption mechanism of HD on Pb（Ⅱ）,to explore the influence of the structural evolution of HC on the mobility and phytoeffectiveness of Pb（Ⅱ）in contaminated soil,and to provide a theoretical basis for the application of remediation technology for heavy metal contaminated soil.The main research results include:（1）The analysis showed that HD was in the form of regular fragments and belonged to the humic acid group,and its particle size in water was around 100 nm,much smaller than that of HC（209μm）;Under acidic conditions,HD was agglomerated and its particle size changed more with time.（2）The adsorption experiments showed that the theoretical adsorption capacity of HD on Pb²⁺in water was 792.64 mg/g,which was much higher than that of HC（46.5mg/g）;the adsorption of HD on Pb²⁺was in accordance with the proposed secondary kinetic curve and the Langmuir adsorption isotherm model,and belonged to monolayer chemisorption;The oxygen-containing functional groups on the surface of HD were studied by characterization means such as FT-IR and XPS.It was found that the ratios of carboxyl（-COOH）and hydroxyl（-OH）groups were significantly increased compared with HC,and these two oxygen-containing groups played a major role in the adsorption of HD.（3）It was found that the adsorption equilibrium system of HC-Pb（Ⅱ）driven by alkalinity was accompanied by the HD dissociation on the surface of HC,resulting in the migration of the HD-Pb（Ⅱ）complex in the water and soil environment;The availability of different forms of Pb（Ⅱ）in the rhizosphere microenvironment was studied through plant hydroponics experiments.It was found that the proportion of effective Pb（Ⅱ）per unit weight of plants in the presence of HC-Pb（Ⅱ）and HD-Pb（Ⅱ）was 0.156 mg/g and0.125 mg/g,respectively,while bw HC-Pb（Ⅱ）was 2.8 and 3.5 times higher than them,although the adsorption capacity of plants for Pb（Ⅱ）in the form of HC-Pb（Ⅱ）and HD-Pb（Ⅱ）was relatively low,However,the environmental risk of Pb（II）in bw HC-Pb（II）retained after HD separation cannot be ignored.Based on the above findings,it was shown that when HC was used for remediation of heavy metal contaminated alkaline soils,the potential plant uptake effect and groundwater contamination risk should be given sufficient attention when Pb（II）adsorbed on hydrochar was released in the form of HD-Pb（II）complexes under alkaline conditions as HD was stripped from hydrochar.