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

Hasara Samaraweera; Abigail Sharp; John Edwards; Charles U Pittman Jr; Xuefeng Zhang; El Barbary Hassan; Rooban Venkatesh K G Thirumalai; Sita Warren; Claudia Reid; Todd Mlsna

doi:10.1016/j.scitotenv.2021.145631

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

Sci Total Environ. 2021 Jun 15:773:145631. doi: 10.1016/j.scitotenv.2021.145631. Epub 2021 Feb 5.

Affiliations

¹ Department of Chemistry, Mississippi State University, Mississippi State, MS, USA.
² Department of Engineering Management and Systems Engineering, The George Washington University, Washington, DC, USA.
³ Department of Sustainable Bioproducts, Mississippi State University, Starkville, MS 39762, USA.
⁴ Institute for Imaging & Analytical Technologies, Mississippi State University, Starkville, MS 39762, USA.
⁵ Department of Chemistry, Mississippi State University, Mississippi State, MS, USA. Electronic address: TMlsna@chemistry.msstate.edu.

PMID: 33940740
DOI: 10.1016/j.scitotenv.2021.145631

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 <150 μm, adsorbent dose 50 mg, 25 mL of 25-1000 ppm phosphate concentration, 24 h, 25 °C). The unwashed CTL exhibited a maximum Langmuir phosphate removal capacity (80.6 mg/g), 5.2-times greater than the washed CTL (15.5 mg/g). Insoluble Ca²⁺ and Mg²⁺ phosphates/hydrophosphate particles dominated CTL's phosphate removal. Phosphates were recovered from both exhausted unwashed and washed CTL better in HCl than in NaOH. P-laden washed CTL exhibited a slow phosphate leaching rate under initial pH of 6.5-7.5 (52-57% over 20 days) after phosphate uptake, indicating it could serve as a slow-release fertilizer. Unwashed CTL retained more phosphates than washed CTL (cumulative q_e for 4 cycles = 391.8 mg/g vs 374.7 mg/g) and potentially improves soil fertility more.

Keywords: Adsorption; Desorption kinetics; Lignite; Phosphate; Surface precipitation; Wastewater treatment.

Published by Elsevier B.V.