Study On The Physicochemical Properties Of Phosphate Copyrolyzed Sewage Sludge-based Biochar

The amount of surplus sludge produced by sewage treatment plants has been increasing year by year,and its treatment problems have become increasingly severe.The sludge contains heavy metals,pathogens,harmful organic components,etc.,as well as a considerable amount of nutrients such as N,P,and K,which has the double-edged properties of"pollution"and"resources".Using sludge to prepare biochar can not only stabilize heavy metals and kill pathogenic bacteria,but also provide nutrients to the soil,which has a good development prospect.Inorganic salt is an inherent component of sludge,and its content will increase due to artificial addition.This research first explores the influence of inorganic salts on the properties of biochar.Inorganic salts containing Ca,Al,Cl,SO₄^2-and PO₄^3-are added to the sludge and then pyrolyzed at 500℃.Affected by the addition of inorganic salts,the ash content,yield and p H value of the sample change.Inorganic salt catalyzes the decomposition of organic matter and reduces the content of C,H,N and S.Some inherent minerals disappear or transform,external inorganic salts and ions in the sludge form new minerals.In the presence of Cl,the retention rate of Cu,Mn,Zn and Pb decreases after pyrolysis.SO₄^2-promotes the volatilization of heavy metals,while PO₄^3-counteracts the effect of Cl.Cl converts the residual speciation of Pb into an exchangeable speciation,SO₄^2-inhibits the conversion of heavy metals from a migrating speciation to a non-migrating speciation,and PO₄^3-increases the proportion of heavy metal residual fraction.Then study the properties of potassium phosphate and sludge co-pyrolysis to prepare biochar and the conversion of P in it.The Hedley fractionation was used to evaluate the effectiveness of P.In co-pyrolysis biochar,the proportion of short-and medium-term available P is～70%,and the ratio of the two is 1:3.4～4.3（short-term:medium-term）.Extrinsic P is distributed in H₂O-,Na OH-and HCl-P pools,the proportions are～30%,～50%and～20%,respectively.The ratio of Na OH-P to iron reached 1:1.7,indicating that iron oxides in the sludge have great potential to combine with P.The binding of extrinsic P is achieved through the formation and adsorption mechanism of iron hydroxyphosphate.Through chemical extraction and instrumental analysis,it was revealed that the phosphorus occluding effect caused by the pyrolysis of iron oxides in the sludge.And phosphate co-pyrolysis alleviated the effect by removing part of the iron oxides and improving the release of inorganic components,which improved the availability of phosphorus.This study revealed the influence of inorganic salts on the properties of biochar and the form of heavy metals,and confirmed the feasibility of phosphate-sludge co-pyrolysis to prepare biochar.Inorganic salt changes the physical and chemical properties of biochar,and affects its fraction through direct and indirect interaction with heavy metals.In the case of adding inorganic salt,its impact on biochar should be given full attention.In the phosphate-sludge co-pyrolysis biochar,most of the extrinsisc P exists in the available form,which not only increases the total amount of available P,but also improves the release of intrinsic P.The P occluding effect caused by the pyrolysis of iron-rich sludge has an adverse effect on the utilization of such sludge.The phosphate co-pyrolysis method used in the study is expected to be an effective way to solve this problem.The results of this research provide theoretical and technical support for the pyrolysis and utilization of sewage sludge.