Phosphorus recovery from the liquid phase of anaerobic digestate using biochar derived from iron−rich sludge: A potential phosphorus fertilizer

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A novel technique for phosphorus recovery from the liquid phase of anaerobic digestate was developed using biochar derived from iron−rich sludge (dewatered sludge conditioned with Fenton’s reagent). The biochar pyrolyzed from iron−rich sludge at a low temperature of 300 °C (referred to as Fe−300 biochar) showed a better phosphorus (P) adsorption capacity (most of orthophosphate and pyrophosphate) than biochars pyrolyzed at other higher temperatures of 500–900 °C, with the maximum P adsorption capacity of up to 1.843 mg g−1 for the liquid phase of anaerobic digestate. Adsorption isotherms study indicated that 70% P was precipitated through chemical reaction with Fe elements, i.e., Fe(II) and Fe(III) 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 using the potassium dihydrogen orthophosphate (KH2PO4) as a model solution. 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. 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−1 at the end of an incubation of 20 days. 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, thus increasing the added values of both the liquid phase of anaerobic digestate and the iron−rich sludge.