The Feasibility Study Of Sewage Sludge Biochar As Adsorbent And Acid Soil Amendment

Sewage sludge is the byproduct of wastewater treatment process, which contains large amounts of organic matter, N, P and other nutrients. However, a variety of toxic substances in sewage sludge also pose a potential threat to environment. In recent years, the disposal difficulty of sewage sludge is increasing with its large quantity. It is reported that pyrolysis is a promising technology to realize the harmless disposal, reduction, stabilization, and reutilization of sludge. Biochar prepared with sewage sludge （SSBC） could be used as an effective adsorbent and soil amendment.In order to systematically investigate the influence of pyrolysis temperature on physicochemical properties, phosphate adsorption potential and soil remediation effect of the sludge biochar, biochars were produced with different pyrolysis temperatures （300,400,500,600, 700 and 750℃）. Results showed that biochar properties were significantly influenced by pyrolysis temperatures. The yield of biochars decreased with the increasing pyrolysis temperature. Biochar produced at low temperature （300℃） was acidic while those at high temperature （higher than 300℃） were alkaline. The contents of C, H and N in the biochar were found to decrease whereas the contents of Ca, Mg, Al, Fe and other heavy metals increased with the increasing pyrolysis temperature. The increase of pyrolysis temperature caused fewer aliphatic functional groups and more aromatic structures.The adsorption capability of NH₄⁺、NO3- and phosphate on biochar were then investigated in this paper. Compared with other different biochar, SSBC showed lower adsorption capability of NH₄⁺and NO3-. However, biochar prepared at 700℃（B700） showed better phosphate adsorption capacity compare with many other carbon-based adsorbents （5.93 mg/g, as P）, which meant that SSBC could be used as a cost-effective adsorbent. Further research demonstrated that phosphate was stably adsorbed on the biochar, and CaO and MgO on biochar surface bonded with phosphate might be the main adsorption mechanism. Batch adsorption experiments showed that the adsorption capacity of biochar increased with the increase of adsorption temperature, the phosphate was effectively adsorbed within a pH range of 6-8, and coexisting CO32+ had significant inhibition effect on phosphate adsorption. Moreover, Langmuir model, Freundlich model, and Langmuir-Freundlich model all fitted the adsorption data well, suggesting that the phosphate adsorption process with biochar B700 was controlled by multiple processes.With the incubation experiment, we found that biochar could significantly increase soil pH, and improve soil organic matter, phosphorus and nitrogen content. The effect is more obvious with the increasing application rate and higher preparation temperature. SSBC contained ample exchangeable base cations and thus SSBC incorporation increased the contents of soil exchangeable Ca2+, Mg2+, K+ and Na+ and decreased soil exchangeable aluminum and H+. In column leaching test, we found that the application of biochar significantly decreased the amount of DOC, NH₄⁺, and phosphate, while the NO3- leaching was not effected.