Preparation And Modification Of Biochar From Agricultural And Forestry Wastes And Its Application In Cd Pollution Control

Biochar is a way for the utilization of waste biomass,which has attracted attention in the field of pollution control because of its advantages of cheap and readily available raw materials,high efficiency of adsorption,and good adsorption potential.Compared with traditional wastewater treatment methods,biochar adsorption can effectively solve the problems of high cost and legacy of pollutants.In addition,due to the enrichment of heavy metals plants,biochar prepared from heavy metal-enriched plants tends to have the risk of introducing secondary pollution during using.In this paper,the preparation and modification of biochar from agriculture and forestry waste and the adsorption of Cd in solution were studied.At the same time,the migration and conversion of Cd and the utilization of char product in pyrolysis of enriched plants were studied.The main results are as follows:Biochar was prepared from five types of agricultural and forestry waste（coconut,rice straw,apricot,willow,ryegrass）and the effect of different raw materials on char yield and adsorption capacity was explored.The rice straw was selected as the best raw material.On this basis,the response surface method was used to optimize the experimental conditions.It was found that the biochar has the best performance when the particle size is 60 mesh,the temperature is 600°C,and the time is 70 minutes.Biomass raw materials were treated with activators.It was found that the effect of biochar after pretreatment with FeCl₃ was best,and the effect was increased by more than 30%.Therefore,the experiment was carried out with rice straw and FeCl₃pretreatment reagent.From the viewpoint of surface oxidation modification,the plasma modification of the rice straw biochar after FeCl₃ pretreatment was performed.It was found that the adsorption capacity of modified carbon for Cd²⁺increased by nearly 20%（to 72.6mg/g）.The adsorption kinetics model satisfies the second-order kinetics model and the adsorption isotherm model conforms to the Langmuir isotherm adsorption model.The adsorption process is endothermic and spontaneous.What’s more,the penetration time of the modified dynamic carbon column increased from 40 h to 140 h,and the adsorption capacity remained above 90%after 8 regenerations.This shows that the Fe-based/plasma-modified biochar has good adsorption potential and regeneration ability.Characterization of the modified carbons revealed that the active functional groups on the Fe-based/plasma-modified biochar significantly increased compared to the unmodified biochar.The complex formed by the Fe-Cd and（-COOH,-OH）functional groups was the key mechanism for adsorption of Cd²⁺.From the viewpoint of surface reduction and modification,amino modification of non-FeCl₃ pretreated rice straw biochar was performed.It was found that the adsorption capacity of modified carbons for Cd²⁺increased by more than 50%（up to55 mg/g）.The adsorption kinetics model satisfies the second-order kinetics model and the adsorption isotherm model conforms to the Langmuir isotherm model.The thermodynamic calculations show that the adsorption is spontaneous perform and absorb heat.Since the specific surface area of the bio-char treated by the amino modification was deteriorated and the adsorption performance became weak,the plasma treatment of the amino biochar was not selected.The dynamic immobilized column study showed that the breakthrough time of amino-modified biochar increased from 40 h to 70 h,and after 8 regenerations,the adsorption capacity was still above 90%.It was found that amino biochar has more amino functional groups on the surface,which is enough for strong complexation with Cd²⁺in solution,and it adsorbs and enriches Cd²⁺in the form of stable complex.After comparing the experimental results,the Fe-based/plasmon modified carbon obtained the better adsorption.And compared with amino-modified carbon,Fe-based/plasma-modified carbon has a high specific surface area and good adsorption potential.Therefore,the Fe-based/plasma-modified method can more effectively improve the biochar adsorption performance and thus efficiently remove Cd²⁺in water.Aiming at the problem of heavy metal enrichment in agriculture and forestry waste,the research on preparation of biochar was carried out,and the problems of Cd migration and conversion and carbon product reuse during pyrolysis were explored.The results show that the distribution of heavy metals in the three components of gas,solid and liquid is mainly affected by temperature.With the increase of temperature,the yield of biochar decreased,the Cd of gas volatiles increased,and the Cd content of charcoal showed a decreasing trend.When the temperature rises above 600°C,the acid-soluble and water-soluble Cd,which are harmful,drop to a very low level.The Cd content in biochar prepared at 800°C is 24.8mg/kg,and the content of the acid-soluble and water-soluble states is 0.372 mg/kg.Adsorption study showed that the maximum adsorption capacity was 25mg/g and the adsorption effect was reduced by 28.6%compared to straw carbon.By reasonably controlling the pyrolysis temperature,the stability of the charcoal product can be achieved.And applying it to the treatment of water pollution can have a certain effect,which provides a reference significance for the post-processing and resource utilization of the enriched plant.