Lignite, thermally-modified and Ca/Mg-modified lignite for phosphate remediation

Aqueous phosphate uptake is needed to reduce global eutrophication. Negatively charged adsorbent surfaces usually give poor sorption. Chemically- and thermally-modified lignite (CTL) was prepared by impregnating low-cost (RL) with Ca2+ Mg2+ cations, basified KOH (pH ̴ 13.9), followed a 1 h 600 °C pyrolysis under nitrogen. CTL has positive surface (PZC = 13) due basic Ca Mg compounds, facilitating the aqueous uptake. CaCO3, MgO, Ca(OH)2, Mg(OH)2 phases 0.22 μm particle sizes were verified XRD, XPS, SEM, TEM, EDX before after Higher amounts of these mineral promoted more than raw thermally treated (TL) without Ca/Mg modification. Phosphorous Ca2+/Mg2+ occurs not classic adsorption but stochiometric precipitation Mg3(PO4)2, MgHPO4, Ca3(PO4)2, CaHPO4. This offers potential substantial capacities. CTL's removal pH-dependent; optimum pH 2.2. Water-washed exhibited maximum Langmuir capacity 15.5 mg/g at 7, 6 14 times higher that TL RL, respectively (particle size <150 μm, dose 50 mg, 25 mL 25–1000 ppm concentration, 24 h, °C). The unwashed (80.6 mg/g), 5.2-times greater washed (15.5 mg/g). Insoluble phosphates/hydrophosphate particles dominated removal. Phosphates recovered from both exhausted better in HCl NaOH. P-laden slow leaching rate initial 6.5–7.5 (52–57% over 20 days) uptake, indicating it could serve as slow-release fertilizer. Unwashed retained phosphates (cumulative qe for 4 cycles 391.8 vs 374.7 mg/g) potentially improves soil fertility more.