The treatment of co-produced coal seam gas water using raw and pre-treated natural ion exchangers

Coal seam gas (CSG) is obtained by pumping water from the saturated coal seam to reduce the pressure allowing the methane gas to desorb. The co-produced water from the gas extraction process has a geochemical signature mainly determined by the moderate levels of salinity, sodicity and dissolved trace elements. Typically, CSG co-produced water requires treatment to be suitable for beneficial re-use, since untreated co-produced water can cause soil infiltration damage and nutritional imbalance for crops and livestock. CSG water from the Bowen Basin (Queensland) was used in this study and has a typical ionic composition of Na+-Cl--HCO3-. In this study, two natural ion exchange materials, zeolite and scoria, were used for the removal of Na+, Sr2+ and Ba2+ from CSG co-produced water. Following XRD analysis, the mineral composition of the zeolite material was found to be consistent of clinoptilolite (41%) and mordenite (29%), while the scoria material presented as diopside (35%), forsterite (33%) and anorthite (29%) characteristic. The real exchange capacity exhibited by the zeolite material was 75 meq/100 g, while the real exchange capacity determined for the scoria was 28 meq/100 g. Adsorption capacity and kinetic rates for Na+ ions were favoured by the use of small fraction sizes of natural forms of the zeolite and scoria material, which increased the accessibility of available adsorption sites on the material for cation interaction, consequently, an optimised fraction size of 0.6 – 0.3 mm was used in all studies. Equilibrium studies used a ratio of 20:1 for solution and material (50 mL : 2.5 g) for 72 h showing that the scoria and zeolite material treated with NH4+ exhibited greater adsorption capacities for Na+, Sr2+ and Ba2+ than the natural form. The maximum Na+ adsorption observed in the scoria and zeolite materials enriched with NH4+ was 17 and 45 meq/100 g, which corresponds to 61% and 60% of the measured real exchange capacity, respectively. The scoria and zeolite materials enriched with NH4+ for Sr2+ adsorption were 4.5 and 8 meq/100 g (44 and 80% removal of the initial Sr2+ concentration). The maximum Ba2+ adsorption achieved using the scoria and zeolite in NH4+ form was 5.8 and 10.6 meq/100 g (40 and 94% removal of the initial Ba2+ concentration). Competitive uptake studies were undertaken to determine selectivity isotherms and coefficients that showed the scoria material’s selectivity series was Ba2+ >Sr2+ >> Ca2+ > K+ > Na+, while, the zeolite material exhibited selectivity for K+ >Ba2+> Sr2+ >> Ca2+ > Na+. Other cations present in CSG water compete with Na+ for adsorption sites, reducing its adsorption capacity to 10 and 38 meq/100 g for scoria and zeolite. The adsorption and desorption of Na+ ions studied in batch mode showed that the ability of scoria and zeolite to adsorb cations decreased with the number of regeneration cycles. The effect of the flow rate on the removal of Na+ by the scoria and the zeolite material in a fixed bed column experiment was relatively small (5% reduction when using flow rates of up 10 BV/h). Columns packed with the scoria and the zeolite material exhibited a larger Na+ dynamic adsorption capacity (breakthrough capacity) when treated with NH4+ as well as larger desorption capacities than the same materials in K+ form. Nonetheless, the adsorption capacity exhibited for both materials was approximately 50% of the real exchange capacity. The maximum bed volumes obtained before the breakpoint (C/C0 = 0.3, output concentration) with a flow rate of 5 BV/h for the scoria material in NH4+ form using CSG water was 1.5 BV, while zeolite in NH4+ form exhibited 4.5 BV. These results showed that in using any natural and pre-treated ion exchange material as a CSG water treatment technology, allow the adsorption of Na+ ions with evident improvement for pre-treated materials. Batch and column type experiments showed that chemical conditioning increased the zeolite and scoria ion selectivity towards cations such as Ba2+, Sr2+ and Na+ present in high concentrations in CSG water. Ion selectivity was found to be correlated to the incoming hydrated cation size and its energy of hydration. The outcomes obtained from the experimentation indicates that natural ion exchangers are suitable for the removal of cations present in CSG water through ion exchange. Nonetheless, limitations to the use of natural and pre-treated ion exchange materials were identified. The high salinity characteristic of CSG water needs to be corrected with other treatment methods or mixed with low salinity water, before the CSG water can be beneficially re-used. The use of natural ion exchange material for the treatment of CSG water may consist of a multi-column arrangement with several trains operating in parallel allowing continuous treatment and facilitate the regeneration of the column without the need to stop the treatment.