Removal of Ions From Produced Water Using Powder River Basin Coal

Although becoming less attractive as an energy source, coal has significant potential for other, more sustainable uses including water treatment. In this study, we present a simple approach to treat water that was produced during oil production and contained a total dissolved solids (TDS) content of over 150 g/L using Powder River Basin (PRB) coal. PRB coal used as packing material in a flow-through column effectively removed 60-80% of the cations and anions simultaneously. Additionally, 71-92% of the total organic carbon in the produced water was removed as was all of the total suspended solids. The removal mechanisms of both cations and anions were investigated. Cations were removed by ion exchange with protons from oxygen-containing functional groups such as carboxylic and phenolic hydroxyl groups. Anions, mainly Cl-1, appeared to be removed through either the formation of resonance structures as a result of delocalization of electrons within coal molecules or through ion-π interactions. We propose that coal is a “pseudo-amphoteric” exchange material that can remove cations and anions simultaneously by exchanging ions with both ionized and non-ionized acids that are ubiquitous in coal structure or resonance effect.