Poultry manure and sugarcane straw biochars modified with MgCl₂ for phosphorus adsorption

Increases in agricultural productivity associated to the crescent use of finite reserves of phosphorus improved the demand for ways to recycle and reuse this nutrient. Biochars, after doping processes, seem to be an alternative to mitigate the large use of P reserves. Sugarcane straw and poultry manure were submerged in an MgCl₂ solution in a 1:10 solid/liquid ratio and subsequently pyrolyzed at 350 and 650 °C producing biochar. Increasing concentrations of P were agitated with biochars in order to obtain the maximum adsorption capacity of P with the aid of Langmuir and Freudelich isotherm. MPAC was extracted, successively, with H₂SO₄ (0.5 mol L^-1), NaHCO₃ (0.5 mol l^-1 a pH 8.5) and H₂O, until no P was detected in the solution. Biochars without the addition of Mg did not have the ability to adsorb P but had this property developed after the doping process. The poultry manure biochar presented higher MPAC (250.8 and 163.6 mg g^-1 of P at 350 and 650 °C, respectively) than that of sugarcane straw (17.7 and 17.6 mg g^-1 of P at 350 and 650 °C, respectively). The pyrolysis temperature changed significantly the MPAC values for the poultry manure biochar, with an increase in the adsorbed P binding energy for both biochars. H₂SO₄ showed the best extraction power, desorbing, with a lower number of extractions, the greater amount of the adsorbed P. These materials doped with Mg and subjected to pyrolysis have characteristics that allow their use in P adsorption from eutrophic and wastewaters and therefore its use as a slow release phosphate fertilizer, indicating to be competitive in quality and quantity with available soluble chemical sources in the market.