Preparation Of MnOx-modified Biochar And Its Immobilization And Retention Effects On Typical Pollutants

The rapid development of urbanization not only provides us with the convenience of life,but also increases the potential risk of pollution in some urban rivers.In some cities with large topographic differences and abundant rainfall,when the rainy season comes,a large amount of rainwater converges to form stormwater runoff.In addition,studies have shown that heavy metals,nitrogen and phosphorus and other plant nutrient pollutants exist in urban stormwater runoff.These pollutants are discharged into urban rivers along with stormwater runoff,and then affect water quality and aquatic ecological environment.Therefore,how to fix and retain pollutants in stormwater runoff has become a hot research topic in water conservation today.In this study,manganese modified biochar was prepared by using traditional agricultural waste rice husk as biomass raw material and modified with Mn（NO₃）₂,and then the physicochemical properties of biochar before and after modification were analyzed.Furthermore,the effect and mechanism of fixing Cr（VI）in water on manganese modified biochar were investigated by the adsorption experiment and XPS analysis technology.On this basis,the soil in the water-level zone along the Yangtze River in the main urban area of Chongqing was taken as the research object,and the indoor soil leaching experiment was carried out to clarify the effect of soil at different depths on the retention of exogenous pollutants in the presence of biochar.It is expected to provide a new experimental and theoretical basis for the application of the manganese modified biochar in the field of heavy metal containing wastewater treatment and the ability to retain typical pollutants carried by stormwater runoff.The main conclusions were as follows:1)The pristine biochar（BC）and manganese modified biochar（Mn OBCs）were prepared by pyrolysis and chemical impregnation,respectively.The physicochemical properties of biochar before and after modification were characterized and analyzed,and the results showed that some particle agglomerates appeared on the surface and in the pores of Mn OBCs,and then XRD analysis showed that Mn OBCs occurred a crystal structure associated with crystalline manganese ore（Mn Ox）.The specific surface area of Mn OBCs is 299.56 m²/g,which is 8.89 times higher than that of BC（33.67 m²/g）.After FT-IR analysis,it was shown that both BC and Mn OBCs contain functional groups such as-OH,C=O,C=C,C-OH,etc.The abundant functional groups can provide reaction sites for the immobilization and retention of contaminants.2)In the experiments of Cr（VI）fixation in water,it was found that the solution p H value had a significant effect on the fixation of Cr（VI）by BC and Mn OBCs,and the adsorption capacity decreased sharply with the increase of p H.The presence of Na Cl had little effect on the fixation of Cr（VI）by BC and Mn OBCs.According to the adsorption kinetic process of BC and Mn OBCs,it was more consistent with Elovich model,indicating that the adsorption process of BC and Mn OBCs on Cr（VI）was mainly through chemisorption and gradually reached adsorption equilibrium after 6 h reaction.In addition,the Langmuir and Freundlich models can better fit the isothermal sorption data,but compared with Langmuir model（R²=0.8852-0.9079）,isotherm data was more consistent with Freundlich model（R²=0.9821-0.9868）.And the maximum adsorption capacity of Mn OBCs for Cr（VI）can reach 28.5824 mg/g,which is 2.83 times higher than that of BC（10.1018 mg/g）.XPS analysis of the material before and after the immobilization of Cr（VI）revealed the presence of Cr（VI）and Cr（III）on the surface of both BC and Mn OBCs,indicating that the Cr（VI）immobilized on the surface of the material was reduced.The percentage of Cr（III）on the surface of Mn OBCs was 44.61%,which was larger than the percentage of Cr（III）on the surface of BC（28.45%）,meanwhile,the Mn2p_3/2 results indicated that part of Mn（II）on the surface of Mn OBCs was oxidized to Mn（III）and Mn（IV）.Moreover,the C1s and O1s results suggested that the functional groups（-COOH and-OH）in Mn OBCs were also involved in the immobilization of Cr（VI）through complexation reactions as well.Thus,the mechanism of Cr（VI）immobilization on Mn OBCs included electrostatic attraction,complexation and redox reactions.3)The effect of BC and Mn OBCs on soil retention of exogenous pollutants（Cr（VI）,TN and TP）was investigated by soil leaching experiments,and it was found that the addition of 5%Mn OBCs to the soil had the best retention effect on exogenous Cr（VI）in both 0-10 cm and 10-20 cm soils,with cumulative retention reaching 0.6244 and 0.6764mg,respectively.It is mainly due to the redox reaction of Cr（VI）by Mn OBCs,which caused the partial reduction of Cr（VI）to Cr（III）.For the retention of exogenous TN,the addition of 5%BC to the soil had the best effect on the retention of exogenous TN,with the cumulative retention amounts reaching 0.5085 and 0.5047 mg,respectively.For exogenous TPs,the order of the magnitude of interception was:control>BC>Mn OBCs.Moreover,it was also found that Mn element appeared in leachate of each soil column,mainly because of the presence of Mn in the soil,BC and Mn OBCs.The highest Mn concentration was found in the drench solution of the soil column with Mn OBCs added,suggesting that the Mn Ox on the surface of the Mn-modified biochar was leached out when reacting with the contaminants.Finally,through the comparison of the size of total retention,the addition of 5%BC in 0-10 cm soil and 10-20 cm soil can effectively improve the effect of soil retention of exogenous pollutants.