Effect Of Acidified Aging On The Physicochemical Properties Of Biochar And Its Effectiveness In Passivating Soil Cd

Biochar prepared from agricultural and forestry wastes applied to agricultural soils can significantly improve soil properties and effectively control the bioavailability and migration behavior of harmful elements.However,the environmental aging effect over a long period of time can change the physicochemical properties of biochar,which in turn affects its ameliorative effect on soil and the passivation effect of harmful elements.In this paper,woody,herbaceous and manure sources were used as biomass raw materials,from which four types of biochar with widely different properties,namely pine biochar（WB）,corn straw biochar（MB）,rice husk biochar（RB）and pig manure biochar（PB）,were selected respectively.Elemental analysis,Specific surface area and pore size analysis（BET）,Fourier transform infrared spectroscopy（FTIR）,X-ray diffraction analysis（XRD）and other characterization methods were used to study the effects of aging effects on the physicochemical properties of biochar by three aging methods,namely water washing,acid rain drenching and acid rain oxidation.On this basis,a pot experiment was conducted to study the improvement effect of biochar aged in different ways on a Cadmium（Cd）-contaminated loam in southwest China,its effect on crop growth and Cd uptake and transport,and to explore the stability of the Cd passivation effect of biochar on soil.The main findings are as follows:（1）The raw materials and aging methods had a greater influence on the physicochemical properties of biochar.Pig manure biochar and rice husk char were dominated by ash material（54.67-56.40%）,while pine char and corn straw char were dominated by C and O elements（57.33-70.65%）,and pig manure biochar had the highest Electrical conductivity（EC）（2.63）and rice husk biochar had the highest p H（9.38）and the smallest EC（0.62）.Acid aging significantly increased the carboxyl and phenolic hydroxyl groups of biochar by 39.78-62.69%,and the specific surface area by 19.27-411.72%.After aging by water washing and acid rain drenching,the crystalline minerals of biochar changed less,depending on the materials.Ash content increased by 1.23-18.91%,while p H and EC significantly decreased by 8.21-19.69%and 56.24-90.37%,respectively;after aging by HNO₃/H₂SO₄oxidation,the crystalline minerals of biochar decreased,and ash content,p H and EC were also significantly reduced by 16.72-67.16%,17-57.04%and 58.94-79.15%,respectively,and the alkaline groups were greatly depleted.（2）Biochar has a significant ameliorative effect on acidic soil and can reduce soil cadmium（Cd）activity,among which the effect of pine biochar and pig manure biochar is more obvious.Acid-aged biochar increased soil organic matter（SOM）content,and the passivation effects on soil p H,soil fertility and soil Cd varied depending on the degree of aging.The application of water-washed and acid rain-drenched aged biochar significantly increased soil p H,EC,cation exchange capacity（CEC）and salt base saturation（SBS）,and reduced soil exchange acid and soil Cd activity,but the addition of HNO₃/H₂SO₄oxidative aged biochar decreased soil fertility and increased the risk of exchange acid and soil Cd activation,indicating that the mild acid aging process of biochar helped to enhance its soil amelioration performance and passivation performance of soil Cd,but the long-term or drastic acid aging process reduced soil p H and soil fertility and promoted soil Cd activation.（3）Biochar can promote the above-ground growth of Brassica chinensis L.and effectively inhibit its uptake and transport of Cd.Pig manure biochar has the most significant and stable effect on the crop.Water-washed and acid rain-drenched aged pig manure biochar increased the above-ground dry weight of Brassica chinensis L.by 2.16-22.19%,and HNO₃/H₂SO₄oxidative aged pig manure biochar reduced the root and above-ground dry weight of Brassica chinensis L.by 10.09-23.53%.According to the accumulation and transport of Cd in Brassica chinensis L.,aging reduces the stability of the passivation effect of all biochar on soil Cd.Acid rain drenching and HNO₃/H₂SO₄oxidative aging may also increase the risk of soil Cd.The results of correlation analysis indicated that the p H,EC,lactone group,alkaline group and H/C of biochar were the key endogenous factors for its ability to mitigate soil acidification,improve soil fertility,and reduce the risk of Cd accumulation and transport in soil exchange acid and soil-plant systems.In summary,the effects of biochar on soil Cd after aging in different ways are different.Pine biochar and pig manure biochar had the best effect on soil improvement and passivation of soil Cd,and pig manure biochar also promoted the above-ground growth of Brassica chinensis L.and reduced the accumulation of Cd transported in Brassica chinensis L.,and the acidic aging effect would weaken the passivation effect of biochar on soil Cd,and the oxidative aging of HNO₃/H₂SO₄might also increase the risk of soil acidification and Cd.