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

Abstract Aqueous phosphate uptake is needed to reduce global eutrophication. Negatively charged adsorbent surfaces usually give poor phosphate sorption. Chemically- and thermally-modified lignite (CTL) was prepared by impregnating low-cost lignite (RL) with Ca2+ and Mg2+ cations, basified with KOH (pH 13.9), followed by a 1 h 600 °C pyrolysis under nitrogen. CTL has a positive surface (PZC = 13) due to basic surface Ca and Mg compounds, facilitating the aqueous phosphate uptake. CaCO3, MgO, Ca(OH)2, and Mg(OH)2 surface phases with 0.22 μm particle sizes were verified by XRD, XPS, SEM, TEM, and EDX before and after phosphate uptake. Higher amounts of these mineral phases promoted more CTL phosphate uptake than raw lignite (RL) and thermally treated lignite (TL) without Ca/Mg modification. Phosphorous uptake by Ca2+/Mg2+ occurs not by classic adsorption but by stochiometric precipitation of Mg3(PO4)2, MgHPO4, Ca3(PO4)2, and CaHPO4. This offers the potential of substantial uptake capacities. CTL's phosphate removal is pH-dependent; the optimum pH was 2.2. Water-washed CTL exhibited a maximum Langmuir phosphate uptake capacity of 15.5 mg/g at pH 7, 6 and 14 times higher than that of TL and RL, respectively (particle size