Australia has rich coal seam gas (CSG) deposits. However, significant quantities of saline water are co-produced and discharged as a by-product of CSG extraction. The untreated CGS water cannot be used due to its high salinity and sodicity because it can potentially increase soil salinity and sodicity when directly used for irrigation. High sodium concentration in CSG water is the major concern and has to be reduced for discharge and usage. This PhD thesis investigates the potential application of zeolites as cheap and effective materials for treating CSG water. Synthetic zeolite (4A and Na-Y) and natural zeolite (clinoptilolite) have been examined, characterized and applied.
Synthetic zeolite 4A and Na-Y are widely used as molecular sieves, catalysts and adsorbents. They are sodium-rich zeolites and have different silicon-to-aluminum ratios. Treatment with sulfuric acid of concentration up to 5 M at room temperature has been attempted to activate the two synthetic zeolite samples. The X-ray diffraction (XRD) studies indicate that the acid treatment can lead to major changes in tetrahedral structures where cations such as sodium are removed and dealumination happens as dissolution progresses. The X-ray photoelectron spectroscopy (XPS) analysis shows the Si/Al atomic ratio increases from 2.94 at 0 M to 8.18 at 0.1 M H2SO4, and a significant binding energy (BE) shift of Si and O occurs. With increasing acid concentration, the acid treatment increases the surface intermediate electronegativity of Na-Y, and the BE of each of main structural element changes in the same way as the electronegativity ratio which is defined as the ratio of each element electronegativity to the total surface electronegativity. Particle size analysis indicates that a recrystallization process occurs during the acid treatment. It is concluded that the synthetic zeolites are not suitable for treating CSG water.
Natural zeolites are effective and economical crystalline aluminosilicate adsorbents and catalysts. The literature survey shows that, of many available natural zeolites, clinoptilolite having high cation exchange capacity and the molecular sieve properties can be suitable for treating CSG water. Clinoptiolite (from NSW) treated and activated using sulfuric acid has examined, characterized and applied in the treatment of CSG water samples from Queensland, Australia. XPS analysis on the near-surface of clinoptilolite particles before and after acid activation reveals significant changes in the atomic % and binding energies of the zeolite chemical elements. With increasing acid concentration, the Si/Al atomic ratio increases from 2.99 at 0 M to 4.92 at 5 M. The BE shifts show a similar trend. High XPS resolution and valence-band spectra show that the BE shifts are influenced by cation removal from the zeolite structure, which results in surface dealumination and increases the efficiency of clinoptilolite in treating CSG water.
Clinoptilolite treated and activated by H2SO4 has also been characterized by surface electrokinetic experiments and analysis. Surface (zeta) potential measurements show that both untreated and acid treated zeolites exhibit negative surface charges and maintain this property in pH 2-10. Surface potential of clinoptilolite samples treated by high acid concentration is less dependent on pH than that of untreated samples or samples treated by low acid concentration. The change in clinoptilolite surface potential is theoretically analyzed applying the Gouy-Stern-Grahame theory on the electrical double layers at solid-solution interfaces. Proton donor-acceptor reactions occurring simultaneously on amphoteric alumina and silica sites, and mass balances are considered and applied to link the clinoptilolite active sites with surface charge and potential. Comparison of the theory with the experimental results shows the important role of the dealumination process occurred by the acid activation, which agrees with the XPS results. The dealumination is the main reason for increasing the surface charge and cation exchange capacity of clinoptilolite after the acid activation
Both synthetic and real CSG waters have been used to examine the effectiveness of treatment using clinoptilolite. The real CSG water samples from Queensland, Australia have been obtained from the Pumphouse Irrigation & Pumping Ltd. The research aims to reduce sodium adsorption ratio (SAR), which is a measure of the relative preponderance of sodium to calcium and magnesium. The results show that the CSG water can be treated using clinoptilolite to remove sodium ions to lower SAR and reduce the pH value. Acid activated clinoptilolite at 30% wt solid ratio can reduce the sodium content from 563.0 to 182.7 ppm, the pH from 8.74 to 6.95, and SAR from 70.3 to 18.5. Based on the results of the batch experiments, the sodium adsorption capacity of the acid-treated clinoptilolite is three times greater than that of the untreated clinoptilolite. These experimental results are supported by the analysis and characterization of clinoptilolite surface charge and dealumination by acid activation. Clinoptilolite, as an abundant and low cost natural zeolite has the potential to become an economic alternative filtration medium for removing excessive Na content in CSG water to lower SAR to the level for environment acceptance.