Screening optimal preparation conditions of low-cost metal-modified biochar for phosphate adsorption and unraveling their influence on adsorption performance

Abundant novel metal-modified biochars have been synthesized for phosphate removal under various preparation conditions, but optimal biochar adsorbents with low cost and high removal performance for application in real scenarios are still largely scarce. In this study, the optimal preparation conditions for metal-modified biochar were screened stepwise by determining their phosphate adsorption performance and cost with frequently used preparation conditions, including different feedstocks (total 7 biomass residues), pyrolysis temperatures (400 °C and 600 °C), modification methods (impregnation and coprecipitation), and modifying metals (Fe/La/Mg). Furthermore, the influences of preparation conditions on adsorption capacity and mechanisms were interpreted via multiple characterization methods by compared with the optimal preparation conditions. The results showed that Mg coprecipitated peanut shell biochar at 600 °C (denoted PS–600–C–Mg) was the optimal phosphate adsorbent with a maximum adsorption capacity of 50.58 mg g−1 and a cost benefit ratio (CBR) of 3.24 × 104 RMB t−1 based on adsorption capacity and cost‒benefit analysis via Monte Carlo simulations. The different mesoporous (2–50 nm) and macroporous structures (>50 nm), which were generated under various preparation conditions and favored the development of metal (hydr)oxides, could explain the greater adsorption capacity of this optimal biochar. Furthermore, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), zeta potential and Fourier transform infrared spectroscopy (FTIR) spectra jointly revealed that modification methods and modifying metal types changed adsorption mechanisms with phosphate significantly. The dominant role of electrostatic force in the adsorption of the optimal biochar towards phosphate was interpreted mainly by the unchanged FTIR spectra, negative surface charge, and further confirmed by the interaction region indicator (IRI) and localized orbital locator (LOL) at the molecular level by using density functional theory (DFT).