Investigation On Preparation And Phosphate Removal Mechanism Of Sludge Biochar Modified Materials

Phosphorus pollution and phosphorus recovery have been a focus of environmental management.As phosphorus emission standards become more strict,adsorption technology shows its advantages in the treatment of low phosphorus concentration wastewater,and finding an efficient and cheap phosphate adsorbent has become more important.Biochar has been regarded as a promising adsorbent,which can achieve high phosphate adsorption ability after modification.Sewage sludge is a potential waste organic material for the biochar preparation and has become a major challenge of environmental issues due to its high water-content,complex composition and increasing output.Carbonizing sludge not only can kill pathogens,degrade organic matters and immobilize heavy metals during the pyrolysis process,but the product biochar can be used for soil amelioration and greenhouse gases adsorption.Therefore,pyrolysis of sewage sludge to produce sludge biochar and further apply in phosphate adsorption is a promising disposal scheme for sewage sludge.However,the sludge biochar is in poor adsorption performance for phosphate anions because of its negative charge on the surface.In order to improve the phosphate adsorption capacity of sludge biochar,this study was designed to load different metals on the sludge biochar in different modification ways according to the properties of sludge and sludge biochar.Different modification methods were firstly tried for sludge biochar.The results showed loading La hydroxide onto the sludge biochar by means of co-precipitation could effectively improve the phosphate adsorption capacity of biochar.The La-modified biochar had uniform adsorption sites on the surface,and was insensitive to competitive anions.Analysis result indicated that chemisorption was the main step during the adsorption process and phosphate was adsorbed by the modified biochar through electrostatic attraction and ligand exchange.The experimental design adopted a solid-solid mixing method to load the metal onto the biochar due to the limitations of the impregnation and co-precipitation methods for the sludge biochar modification,that is,mixing the raw materials first and then pyrolysis for biochar preparation.The study firstly compared the adsorption performance of sludge biochar modified by mineral（dolomite）and chemical reagent（basic magnesium carbonate）,which were recorded as DSS（dolomite modified biochar）and MSS（basic magnesium carbonate modified biochar）.The experiment results showed that the isothermal adsorption processes of DSS and MSS could be better described by the Langmuir model,and the maximum adsorption capacity of the two adsorbents at the temperature of 25?C were 29.18 mg/g and 148.66 mg/g,separately.Adsorption kinetic data were all in line with pseudo-second-order kinetic model.Meanwhile,the phosphate adsorption capacity of DSS and MSS were higher under acidic condition.The competitive anions SO₄^2-、HCO₃^-in wastewater had negative effect on phosphate adsorption in different degrees for four adsorbents.In order to further reduce costs,egg shells and oyster shells were tried as raw materials to prepare phosphate adsorbents,namely ESS（egg shell modified sludge biochar）and OSS（oyster shell modified sludge biochar）.The results displayed the isothermal adsorption process of ESS was consistent with the Langmuir model,while the OSS fitted the Freundlich model better.The maximum adsorption capacity of the two adsorbents at25?C were 154.18 mg/g and 129.03 mg/g,respectively.Adsorption kinetic data of ESS and OSS could be better described by pseudo-second-order kinetic model.The optimal pH for phosphate adsorption using ESS and OSS was 9 and the adsorption process could be easily affected by the existing of anions SO₄²-and HCO₃-.Economic analysis of the four biochars shows that the cost of phosphate removal is affected by many factors,among which the cost of the modified material is the most significant.The cost of phosphate removal by the four biochars is about 3.65$/kg,12.65$/kg,0.36$/kg and 2.72$/kg separately.Therefore,the use of egg shell to modify the sludge biochar is the best and cheapest,and has a good application prospect.