Pyrolyzed Ca-Impregnated Lignite For Aqueous Phosphate Removal: Batch And Column Studies

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 degrees C to prepare A-L, improving its surface area by-984-fold. Ca-L (27 wt% Ca) showed a large phosphate uptake (227.3 mg/g) (adsorbent dose 50 mg, 25 mL of 10-1500 ppm phosphate, 24 h, 25 degrees 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/Ca-3(PO4)(2). The breakthrough capacity of a 2.0 g Ca-L bed column (bed height 2.5 cm, diameter 1 cm) was 58.2 mg/g (flow rate 1.5 mL/min, 25 degrees C, initial [PO43-] = 46.6 mg/L, particle size, 125-150 mu m),-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.