Metal-modified biochars prepared from blue algae and their ability of adsorbing phosphates from water and utilization as soil amendment

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The problem of blue algae accumulation can be alleviated by utilizing blue algae as biomass for resource utilization, which can remove excess phosphates from water to alleviate eutrophication. Herein, Magnesium (Mg) and/or lanthanum (La) were used to modify biochar via high-temperature slow pyrolysis with blue algae as the raw biomass. These modified biochars exhibited abundant carbon (18.0 %-50.3 %), oxygen content (17.4 %-49.1 %), along with a high specific surface area (82.26–233.80 m2/g). The surface of metal-modified biochar was enriched with hydroxyl and carboxyl groups and metal elements were efficiently incorporated into biochars. Adsorption kinetic and isotherm experiments were conducted under the optimal pyrolysis temperature of 600 °C, pH of 6.0, and adsorbent dosage of 2.0 g/L. The adsorption kinetics studies by metal-modified biochars for phosphate fitted well pseudo-second-order model Redlich-Peterson isotherm model. Biochar modified by Mg and La exhibited the highest removal efficiency (97.8 %) for phosphate at adsorption equilibrium. The adsorption mechanism involved the electrostatic interaction between hydroxylated metal oxides and negatively charged phosphate ions and the precipitation of phosphate ions through chemical reactions with metal oxides in the solution. After phosphate adsorption, the pristine and metal-modified biochars derived from blue algae were applied as the soil amendment. Biochar adsorbed phosphate apparently improved the germination rate of seeds facilitated the height, width, weight, and chlorophyll content of vegetable seedlings, and induced oxidative stress, which proved that the biochar recovered after adsorption was a high-quality soil amendment. This study provides a new approach for the treatment of phosphate wastewater and the algae application.