Study On The Adsorption Of Ammonia And Phosphate By Biochar

Nitrogen and phosphorus are the main nutrient elements lead to eutrophication,so the removal of ammonia and phosphate has an important role in controllingeutrophication. Of the commonly used methods for removing ammonia and phosphate,adsorption is fast, efficient, easy to operate, no secondary pollution, recyclable andlow cost. There is a lot of wheat straw in China, the rational use of wheat straw canavoid wasting resources, reduce waste emissions and protect the environment. In thisstudy, wheat straw was made into biochar at different temperatures, and modified byferric chloride. This study focused on the adsorption of ammonia and phosphate bybiochar and modified biochar, and the effects of temperature, dosage and pH on theadsorption. The phosphorus forms in biochar were also studied. The main conclusionsare as follows:The cation exchange capacity (CEC) of biochar decreased with the increase ofpyrolysis temperature. The second-order kinetic model showed the best ft to theexperimental data. Langmuir isotherm could describe the adsorption equilibrium; themaximum adsorption capacity decreased with the increase of pyrolysis temperature,and showed a positive correlation with the CEC. The removal rate of ammonia raisedwith the increase of temperature. The Gibbs free energy change was negative, theenthalpy change was positive, and the entropy change was positive, which showedthat the adsorption process was spontaneous, endothermic, and the degree of chaosincreased. The removal rate of ammonia raised as the dosage of biochar increased.With the increase of pH, the removal rate initially increased and then decreased, themost suitable pH was around8.The biochar modified by ferric chloride (M-400-Fe) showed the highestadsorption capacity of phosphate, the adsorption capacity was4.685mg/g,12.33times of the biochar before modification (M-400). The second-order kinetic modelshowed the best ft to the experimental data of M-400and M-400-Fe. Langmuirisotherm could describe both of the adsorption equilibrium; the maximum adsorptioncapacity of M-400-Fe was10.12mg/g,19.42times of M-400. The removal rate ofammonia by M-400-Fe decreased with the increase of temperature. The Gibbs freeenergy change, enthalpy change, and entropy change were all negative, which showed that the adsorption process was spontaneous, exothermic, and the degree of chaosdecreased. The removal rate of ammonia raised as the dosage of biochar increased.With the increase of pH, the removal rate initially increased and then decreased, themost suitable pH was around7.The content of exchangeable phosphorus in M-400increased most afteradsorption, account for82.1%of the total increment. The content of exchangeablephosphorus, iron-bound phosphorus and occluded phosphorus in modified biocharincreased after adsorption, account for26.6%~33.5%,50.7%~66.7%and7.2%~16.2%of the total increment respectively. The content of exchangeable phosphoruswas the highest when the initial pH was7. With the increase of pH, iron-boundphosphorus had a potential of transforming into occluded phosphorus.