Phosphorus removal from eutrophic water using modified biochar.

Increasing problems related to water eutrophication, commonly caused by the high concentration of phosphorus (P), are stimulating studies aimed at an environmentally safe solution. Moreover, some research has focused on the reuse of P due to concerns about the end of its natural reserves. Biochar appears to be a solution to both problems and may act as a recovery of eutrophic/residual water with the subsequent reuse of P in agriculture, the purpose of which is to test such an assertion. Samples of biochar from poultry manure (BPM) and sugarcane straw (BCS) had their maximum adsorption capacities of Al obtained by Langmuir isotherm. These values were used to conduct the so-called post-doping process, conferring P adsorption capacity to the pyrolysed materials. Langmuir and Freundlich isotherms were adjusted for the same biochar types (Al-doped) at increasing P concentrations, in order to obtain their maximum P adsorption capacities (MPAC) and their parameters. The desorption of the adsorbed P in its MPAC was tested by three extractors: H2SO4, NaHCO3, and H2O. Finally, these biochars were used in competitive adsorption assays of phosphate, sulfate, chloride and nitrate anions and applied in a synthetic eutrophic water. The high values of MPAC of the powder materials (701.65 and 758.96mgg-1 of P for BPM and BCS, respectively) are reduced by almost half for the fragment materials (356.04 and 468.84mgg-1 of P for BPM and BCS, respectively), these values being almost entirely extracted the extractors. Its application in eutrophic/residual water, in addition to presenting a good MPAC, these materials adsorbed, in equal proportions, phosphates and sulfates, as well as to a lesser extent, nitrates and chlorides. Thus, biochar from poultry manure and sugarcane straw, after post-doping with Al, have high MPAC, being excellent materials for the recovery of waters and subsequent reuse in agriculture.

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.