Lanthanum and cerium functionalised forestry waste biochar for phosphate removal: Mechanisms and real-world applications

Phosphorus as a diminishing resource, is an essential element for the growth of biota. However elevated concentrations in water imbalance the ecosystem and cause eutrophication. To address both problems, two highly efficient biochars, namely La-OB and CeLa-OB were synthesised by lanthanum and cerium doping. The performance of La-OB and CeLa-OB were evaluated in both, batch and dynamic regimes with synthetic phosphate (PO43--P) solutions and eutrophic lake water. The PO43--P adsorption mechanisms were investigated in terms of pH, kinetics, isotherms, activation energy, thermodynamics and in-depth characterisation (XPS, FTIR, XRD, BET, pHpzc and SEM-EDX); the feasibility of using biochar in 'real life' was studied with column filtration of lake water with low and high PO43--P concentrations, along with reusability/desorption and toxicity trials in the water environment. The mechanistic study showed that both biochars dominantly made inner-sphere complexes with La and Ce ligands (as monolayer), while outer-sphere complexation was less common. The exothermic adsorption process followed Elovich kinetic and Langmuir isotherm models with maximum adsorption capacity of 112 mg/g for La-OB and 40 mg/g for CeLa-OB. In column filtration trials the adsorption capacity reached 78 mg/g, while low metal leaching (0.08 mg/L) and low toxicity to Lepidium sativum germination were detected. The results indicated that these biochars, especially La-OB could be used in circular economy-based water treatment.