Application of the engineered sewage sludge-derived biochar to minimize water eutrophication by removal of ammonium and phosphate ions from water

Abstract

Engineered biochars obtained by pyrolysis of sewage sludge (SSL) are interesting materials that are worth investigating both in terms of nutrients adsorption and their re-usage in agriculture. In the present study, single- or bi-solute adsorption of phosphate and ammonium ions on biochars was performed. The biochars were produced from SSL or from SSL and willow (6:4 w/w) at temperatures of 500, 600, and 700 °C. During the pyrolysis process, two different carrier gases were used: N2 or CO2. After adsorption, desorption of the studied ions was carried out using deionized water (DI). The Langmuir isotherm (R2 > 0.805) best described the adsorption of phosphate ions on all biochars and ammonium ions on the biochars produced from SSL alone. Willow addition to SSL caused the change in the ammonium ions sorption mechanism to Freundlich model (R2 > 0.861). The SSL-derived biochar (700 °C; N2 atmosphere; single-solute adsorption) was characterized by the highest theoretical phosphate ions sorption capacity (88.1 mg/g), which was associated with development of the specific surface area (SBET) and a high content of the inorganic fraction and metals. The SSL and willow-derived biochar (500 °C; N2 atmosphere; single-solute adsorption) had the highest ability to bind ammonium ions (36.0 mg/g), which was dependent on the density of functional groups. Desorption of ions was dependent on adsorption mechanism, physico-chemical properties of biochars and adsorption system. The maximum desorption of phosphate ions (44.8%) was observed for SSL-derived biochars (bi-solute adsorption; 500 °C), while in the case of ammonium ions (20.1%) for the SSL and willow-derived biochars (bi-solute adsorption; 700 °C). Replacing N2 with CO2 during pyrolysis caused higher desorption of phosphate and ammonium ions. Thus, re-use of SSL and willow-derived biochar produced at N2 atmosphere (after adsorption process) in agriculture may regulate the mobility and availability of nutrients (particularly in the case of phosphate ions), due to their lower but longer release rate compared to biochar from SSL alone.