Pyrolyzed Ca-impregnated lignite for aqueous phosphate removal: Batch and column studies

Abstract

Abstract Lignite is an abundant carbon material with a variable surface structure and low cation density The introduction of metal (hydr)oxide phases has improved the anionic binding potential of lignite. In this study, activated lignite (A−L), Ca2+−modified lignite (Ca−L), and Ca2+−modified activated lignite (Ca−A−L) were synthesized to remove aqueous phosphate. Lignite was first activated (KOH: lignite, mass ratio, 3:1) at 750\xa0oC to prepare A−L, improving its surface area by ~984-fold. Ca−L (27\xa0wt% Ca) showed a large phosphate uptake (227.3\xa0mg/g) (adsorbent dose 50\xa0mg, 25\xa0mL of 10–1500\xa0ppm phosphate, 24\xa0h, 25\xa0°C, initial pH 6), due to the large amounts of micro-sized CaCO3, Ca(OH)2, and CaO particles in Ca−L. These particles actively precipitate phosphate/hydrophosphate as CaHPO4/Ca3(PO4)2. The breakthrough capacity of a 2.0\xa0g Ca−L bed column (bed height 2.5\xa0cm, diameter 1\xa0cm) was 58.2\xa0mg/g (flow rate 1.5\xa0mL/min, 25 oC, initial [PO43-] = 46.6\xa0mg/L, particle size, 125–150\xa0µm), ~4 fold lower than the maximum Langmuir sorption capacity. An interference study indicated that Ca−L is highly selective for phosphate. Spent Ca−L may improve soil fertility as it retains more phosphate species for later slow-release to the soil. Unit weight of phosphate can be removed by Ca−L more inexpensively than Norit ROW and Darco KB (two commercial activated carbon carbons). Precipitated Ca2+ phosphates/hydrophosphates in exhausted Ca−L can be recovered using HCl and Ca−L recycled. Moreover, low-cost lignite is a promising carbon support for the future synthesis of different value-added products.