Transformation Of Phosphorus Species During The Co-pyrolysis Of Phosphorus Rich Solid Waste And Biomass And Study On Leaching Characteristics Application

The crisis of phosphorus resources and phosphorus pollution coexist in the world.The production of phosphorus fertilizer from solid waste containing phosphorus is an important way to improve environment and recover phosphorus resources.The co-pyrolysis of solid waste containing phosphorus and biomass to produce carbo-based phosphate fertilizer has received considerable attention,which can solve the problem of phosphorus resource recovery and further improve the utilization efficiency of phosphorus.The effectiveness of carbon-based phosphate fertilizer depends on the source and treatment of solid waste.At present,the fertilizer efficiency of carbon-based phosphate fertilizer produced from the solid waste sources containing phosphorus mainly focused on by researches.However,there are few researches on the transformation of morphological phosphorus and the bioavailability of corresponding phosphorus.Investigation on this aspect has important theoretical significance and practical application value.Initially,aluminum phosphate was selected as the model compound for phosphorus-rich solid waste.Experimental results showed that the phosphorus enrichment rate in biochar increased with increasing pyrolysis temperature.The enrichment ratios of sodium-doped and calcium-doped biochars reached their maxima at 82.6%and 79.5%,respectively,when pyrolyzed at 800℃.The phosphorus enrichment ratios of magnesium-doped and pristine biochars reached their maxima at 81.7%and 75.9%,respectively,when pyrolyzed at 1000℃.With the increase in pyrolysis temperature,the inorganic phosphorus（IP）and total phosphorus（TP）in the biochar increased,while the organic phosphorus（OP）converted to IP,and Al-P converted to Ca-Na-P,Ca-P and Mg-P converted to AP hydroxyapatite.The contents of AP hydroxyapatite and AP/IP in the biochars gradually increased.Sodium biochar’s AP/IP ratio at 1000℃was almost 1.The addition of alkaline earth metals in biochar improved the effectiveness of phosphorus with increasing temperature.The conversion behavior of available P in biochar at 1000℃is affected by alkaline earth metal factors.The phosphorus bioavailability in sodium-doped,calcium-doped,and magnesium-doped biochars at 1000℃ was 1.74,2.10 and 1.78 times,respectively,higher compared to that of pristine biochar.The addition of sodium salt hindered the thermal decomposition of hemicellulose in the biomass,which resulted in decreased specific surface area and pore size of the biochar.In contrast,the addition of magnesium salt promoted the development of porous structure in the biochar,which significantly enhanced the specific surface area and pore volume.Based on the above conclusions,the phosphorus conversion behavior in solid waste thermal hydrolysis carbon was deeply investigated.It was found that the phosphorus enrichment rate of sludge and biogas residue co-pyrolysis carbon increased with the increase of sodium salt addition.Further analysis of the phosphorus form revealed that both monoesters and diesters of orthophosphate in sludge and digested sludge were converted to orthophosphate during high temperature pyrolysis,resulting in extremely low inorganic phosphorus（IP）content and nearly identical to total phosphorus（TP）content in the co-pyrolysis char.The effect of sodium salt addition on IP was not significant,suggesting the possible conversion from pyrophosphate to orthophosphate and the gradual reduction of non-apatite inorganic phosphorus（NAIP）with the increase of sodium salt addition.The effective phosphorus proportion in the char obtained from single pyrolysis of sludge and digested sludge increased from 225.25mg and 171.9mg to a maximum of 340.45mg and217.8mg,respectively,with the increase of sodium salt addition.The temperature of pyrolysis was increased from 600℃to 1000℃.Sodium addition in co-pyrolysis of sawdust and sludge char led to the conversion of Ca-Al-P to Ca-Na-P,while in co-pyrolysis of sawdust and digested sludge char,Ca-Fe-P disappeared and Ca-Na-P and Ca-Mg-P were generated.Partial transformation of Ca-P to Ca-Na-P was also observed in char from single pyrolysis.The role played by biomass in high temperature pyrolysis at 1000℃may be through the reduction of Ca₁₈Mg₂（H₂PO₄）₁₄ to Ca-Mg-P.The study further explored the release behavior of phosphorus from pyrolysis char of phosphorus-rich solid waste through leaching experiments.The experimental results indicated that the addition of sodium not only promoted the effective phosphorus content in the pyrolysis char of the phosphorus-rich solid waste,but also significantly increased the water-soluble phosphorus content in the char and the ratio of CA-extractable water-soluble phosphorus to total effective phosphorus.The releasing tendency of CA from co-pyrolysis char of wood chips and sewage sludge decreased compared with that of single-material pyrolysis char,possibly because the co-pyrolysis char of wood chips and sewage sludge had the characteristics of biochar and a p H higher than that of single-material pyrolysis char.The leaching of H⁺ions and the addition of sodium may have formed sulfates,resulting in a low p H during the first 12 days and an increase in EC electrical conductivity.The presence of soluble monocalcium phosphate accelerated the release of phosphorus in the pyrolysis char containing it compared with other pyrolysis char.The co-pyrolysis char exhibited similar performance at p H 9 and 7,while the release of phosphorus increased after 12 days for the co-pyrolysis char at p H 5 due to the deprotonation of functional groups and the dissolution of some acidic soluble phosphorus.This article explored the effects of alkali and alkaline earth metals added to poplar sawdust on the transformation of phosphorus-rich solid waste.The migration and transformation mechanism of phosphorus during the pyrolysis of alkali and alkaline earth metal-doped biochar was proposed.The conversion behavior of phosphorus-rich solid waste sludge and biogas residue during co-pyrolysis with sodium-salt-doped wood sawdust was elucidated,providing theoretical guidance for the application of phosphorus-rich solid waste-based biochar fertilizer.