Study On The Transformation Of Phosphorus And Reuse Of The Fe-rich Biochar Pyrolyzed From Fe-rich Sludge

The treatment and disposal of sewage sludge is a universal challenge.And the high content of bound water in sewage sludge posed a big challenge for the deep sludge dewatering.Fenton conditioning can effectively remove the bound water of sludge and improve the sludge dewaterability.However,a large amount of iron-rich sludge was inevitably generated during sludge dewatering conditioned with Fenton’s reagents.The inproper disposal of iron-rich sludge will not only lead to environmental pollution,but also result in an unavoidable waste of iron resources.In this study,the iron-rich sludge from the dewatered cake of sludge conditioned by Fenton’s reagents was pyrolyzed to obtain Fe-rich biochar.The transformation of phosphorus in biochar pyrolyzed at different temperatures was analyzed.Meanwhile,the utilization of iron-rich biochar to reclaim P from the liquid phase of anaerobic digestate was studied and the related feasibility of using P-adsorbed biochar as a prospective P fertilizer to promote seed germination was investigated as well.The mechanism of synergistic removal of Cr（VI）by iron-rich biochar and Pseudomonas aeruginosa was investigated.The main contents are as follows:1.The P transformation and speciation in Fe-rich biochar pyrolyzed at different temperaturesThe P transformation and speciation in study at different pyrolysis temperatures were evaluated.The results showed that both the pyrolysis temperatures and the content of Fe in sludge had significant effects on the total phosphorus content and the distribution of phosphorus species.When the pyrolysis temperatures increased from 300 ^oC to 900 ^oC,the total phosphorus content of Fe-rich sludge biochars was higher than that of the raw sludge biochars（18.06-36.26 mg/g versus 15.54-30.59 mg/g）.Fe-rich biochar was predominant with Na OH-P,this can be related with the relatively high iron content in Fe-rich sludge.However,the raw sludge biochar was predominant with HCl.Liquid ³¹P Nuclear Magnetic Resonance(³¹P-NMR)spectroscopic analysis results showed that raw sludge biochar was mainly consisted of ortho-P and pyro-P.The P species were indiscernible for raw sludge pyrolyzed at 800-900°C and all the biochars pyrolyzed from Fe-rich sludge.It can be deduced that the high predominance of iron contents therein,thus interfering the ³¹P-NMR analysis.The results of X-Ray Diffractometry（XRD）analysis showed that both raw sludge biochar and Fe-rich biochar contained a certain amount of iron phosphate at the pyrolysis temperature of 800 ^oC and 900 ^oC.The plant availabilities of Fe-rich biochar and raw sludge biochar were studied by citric acid extraction.Moreover,the plant-availability phosphorus content of Fe-rich biochar was higher than that of the raw sludge biochar under the same pyrolysis condition during the pyrolysis process at 300-800 ^oC.2.Adsorption of phosphorus from the liquid phase of anaerobic digestate by using Fe-rich biochar derived from sludgeUsing Fenton-conditioned iron-containing sludge as raw material,preparing of iron-rich biochar adsorbent by pyrolysis.A novel technique for phosphorus recovery from the liquid phase of anaerobic digestate was developed.The results showed that the biochar pyrolyzed from iron-rich sludge at a low temperature of 300 ^oC（referred to as Fe-300 biochar,the Fe-rich sludge pyrolyzed at 300 ^oC）showed a better P adsorption capacity than biochars pyrolyzed at other higher temperatures of 500-900 ^oC,with the maximum P adsorption capacity of up to 1.843 mg/g for the liquid phase of anaerobic digestate.It can be deduced that the P adsorption by the Fe-300 biochar may be not determined by the physical adsorption alone.Since a high abundance of Fe element（total iron content of 84.37 mg/g Dry Solids（DS））existed in the Fe biochar,it was hypothesized that the chemical reactions between Fe species and P may also contribute to the higher TP adsorption efficiency.Combined with ³¹P-NMR analysis,Fe-300 biochar was capable in adsorbing orthophosphate and pyrophosphate.Using the potassium dihydrogen orthophosphate（KH₂PO₄）as a model solution,the adsorption kinetics of phosphorus were consistent with the pseudo-second-order model,the adsorption isotherms were consistent with Langmuir model.These results indicated that chemisorption mechanism may be involved in the adsorption of phosphorus by the Fe-300 biochar.And adsorption isotherms study indicated that 70%P was precipitated through chemical reaction with Fe elements,e.g.,Fe²⁺and Fe³⁺existed on the surface of the Fe-300 biochar,and other 30%was through surface physical adsorption as simulated by a dual Langmuir-Langmuir model.The results of XRD analysis demonstrated the existence of iron hydrogen phosphate.The final desorption efficiency of still more than 97%of the adsorption capacity after 5 successive adsorption/desorption cycles.Exploring the feasibility of Fe-rich biochar as a slow-release fertilizer through grass seed germination and early stage seedling growth experiments.The results showed that the seed germination rate was increased up to 92%with the addition of Fe-300 biochar after adsorbing most of P,compared with 66%without the addition of biochar.The shoot length increased from 3.1 cm to 5.8 cm.Moreover,P adsorbed by the chemical reaction in form of iron hydrogen phosphate can be solubilized by a phosphate-solubilizing microorganism of Pseudomonas aeruginosa,with the total solubilized P amount of 3.045 mg/g at the end of an incubation of20 days.Combined with P species analysis,it was concluded that the predominant P species were orthophosphate and pyrophosphate from the Fe-300 biochar released to the liquid phase.3.Synergistic removal of Cr（VI）by iron-rich biochar and Pseudomonas aeruginosaUsing Fe elements existed on the surface of the Fe-300 biochar as a reducing agent,a synergistic removal of Cr（VI）with phosphate-solubilizing microorganisms,namely Pseudomonas aeruginosa was studied.The results indicated that the Fe element existed on the surface of the Fe-300 biochar was more likely to reduction Cr（VI）in the neutral environment.Combined Fe-300 biochar with reductase from Pseudomonas aeruginosa,the removal rate of Cr（VI）was over 80%,and Cr（III）was main reduction product.Moreover,Pseudomonas aeruginosa can promote the release of P from Fe-300 biochar to the liquid phase according to the stability of biochar,so that phosphate and Cr（III）formed chromium phosphate precipitation on the surface of biochar.Meanwhile,Cr was passivated to reduce the dissolution rate.Moreover,with the chemical reaction between Fe²⁺and Cr（III）,the H⁺will be consumed in the solution,making the p H of whole system being alkaline,thus leading to the formation of chromium hydroxide precipitation and reducing the content of Cr（III）in the solution.This study indicated that the iron-rich sludge-derived biochar could be used as a novel and beneficial functional material for P recovery from the liquid phase of anaerobic digestate.The recovered P with biochar can be re-utilized in garden soil as an efficient P-fertilizer to promote seed germination.The feasibility of synergistic removal of Cr（VI）by iron-rich biochar and Pseudomonas aeruginosa is also studied.This study inspires the development of new idea and provides experimental basis for the resource utilization of iron-rich sludge.