Application and mechanism of nucleation-induced pelleting coagulation (NPC) in treatment of fracturing wastewater with high concentration of dissolved organic matter

Fracking wastewater treatment: Catalytic performance and life cycle environmental impacts of cerium-based mixed oxide catalysts for catalytic wet oxidation of organic compounds

Water scarcity and the consequent increase of freshwater prices are a cause for concern in regions where shale gas is being extracted via hydraulic fracturing. Wastewater treatment methods aimed at reuse/recycle of fracking wastewater can help reduce water stress of the fracking process. Accordingly, this study assessed the catalytic performance and life cycle environmental impacts of cerium-based mixed oxide catalysts for catalytic wet oxidation (CWO) of organic contaminants, in order to investigate their potential as catalysts for fracking wastewater treatment. For these purposes, MnCeO_x and CuCeO_x were tested for phenol removal in the presence of concentrated NaCl (200 g L^-1), which represented a synthetic fracking wastewater. Removal of phenol in pure ("phenolic") water without NaCl was also considered for comparison. Complete (100 %) phenol and a 94 % total organic carbon (TOC) removal were achieved in both the phenolic and fracking wastewaters by utilising MnCeO_x (5 g L^-1) and insignificant metal leaching was observed. However, a much lower activity was observed when the same amount of CuCeO_x was utilised: 23.3 % and 20.5 % for phenol and TOC removals, respectively, in the phenolic, and 69.1 % and 63 % in the fracking wastewater. Furthermore, severe copper leaching from CuCeO_x was observed during stability tests conducted in the fracking wastewater. A life cycle assessment (LCA) study carried out as part of this work showed that the production of MnCeO_x had 12-98 % lower impacts than CuCeO_x due to the higher impacts of copper than manganese precursors. Furthermore, the environmental impacts of CWO were found to be 94-99 % lower than those of ozonation due to lower energy and material requirements. Overall, the results of this study suggest that the adoption of catalytic treatment would improve both the efficiency and the environmental sustainability of both the fracking wastewater treatment and the fracking process as a whole.

Effects of suspended particulate matter from natural lakes in conjunction with coagulation to tetracycline removal from water

Coagulation is a common method used to remove suspended particulate matter (SPM) from the water supply. SPM has preferable adsorption ability for antibiotics in water; therefore, SPM adsorption and coagulation may be a possible way to remove tetracycline (TC) from water. This study carried out coagulation experiments combining SPM collected from a natural lake at a location with three common coagulants-polyaluminum sulfate, polyaluminum chloride, and polyferric sulfate-under different pH values, exploring the adsorption of TC by SPM, coagulation of SPM with TC, and the primary influencing factors of this process. The maximum removal rate of TC can reach 97.87% with an SPM concentration of 1000 mg/L. Multi-factor analysis of variance showed the importance of various TC removal factors, which were ranked as follows: SPM concentration ≫ initial TC concentration > type of coagulant > pH values. The higher the SPM concentration, the better the TC removal (p < 0.001). Fourier infrared spectroscopy results demonstrated the strong adsorption effect of SPM on TC after being combined with a coagulant, and scanning electron microscopy also indicated that SPM becomes effective nuclei in the coagulation process, which is a possible reason for better TC removal. However, the effluent turbidities under 1000 mg/L SPM concentrations were high without coagulant aid. With the addition of coagulant aid anion polyacrylamide, the TC removal remained unchanged, effluent turbidity significantly reduced, and the TC desorption became low. These results indicate that applying SPM from natural lakes in the coagulation process could potentially remove TC in water.

Mechanism of fluoride removal by AlCl3 and Al13: The role of aluminum speciation

Coagulation is an important defluorination process. However, because of the poor sedimentation properties, conventional coagulants often result in limited defluorination performance and excessive residual aluminum. In this study, AlCl₃ and the highly-positively-charged molecule [AlO₄Al₁₂(OH)₂₄(H₂O)₁₂]⁷⁺ (Al₁₃) was utilized to treat fluoride-containing water. By comparison, the role of aluminum speciation in fluoride removal was elucidated. Under initial pH of 6.0, 7.0 and 8.0, the highest defluorination efficiencies of high-fluoride water ([F^-]₀ = 8.0 mg/L) were 78.2%, 71.6% and 83.2% at Al₁₃ dosage of 20 mg/L, 40 mg/L and 50 mg/L. Combined with detailed investigations of the chemical compositions of flocs, along with electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS) analysis of residual aluminum, the defluorination mechanisms of both coagulants were discussed. In acidic conditions, both coagulants hydrolyzed and formed various Al clusters, among which transient Al (Al_ts) was the intermediate of the other clusters. The coprecipitation of high-polymerized Al and F^- contributed most of the defluorination rate. While under neutral and alkaline conditions, hydrogen bonding and ion exchange together with coprecipitation were the main roles for Al₁₃. The effects of AlCl₃ were merely physical actions, and were affected by the decreased pH. This work provides new insights into the coagulation and defluorination process.

Humic Acid Removal from Water with PAC-Al30: Effect of Calcium and Kaolin and the Action Mechanisms

Polyaluminum chloride with a dominant species of Al30 (PAC-Al30) was prepared in laboratory and used for humic acid (HA) removal from water. The action properties and mechanisms of PAC-Al30, HA, calcium, and kaolin were tested and discussed. The results showed that the existence of calcium or kaolin contributed to the HA removal when the PAC-Al30 dosage was deficient and had no obvious effect when the amount of PAC-Al30 was sufficient. When the PAC-Al30 dosage was 0.01 and 0.02 mmol/L, the HA removal rate was increased by 66.59 and 42.20%, respectively, with a calcium concentration of 2.0 mmol/L, or increased by 53.31 and 40.92%, respectively, with the kaolin particle concentration of 150 mg/L. Calcium could compress the double electrical layers or complex with HA to neutralize a part of the surface negative charge of HA, but could not make the water system reach its isoelectric point. The mechanisms of calcium and kaolin's promoting coagulation effect were adsorption neutralization and collision aggregation respectively, but these actions were much weaker than that of PAC-Al30 with HA. The adsorption neutralization capacity of PAC-Al30 was calculated to be nearly 60 times than that of calcium, and the higher γ value of calcium modified by the Sips equation may indicate that the adsorption or neutralization sites of calcium on HA were pickier than PAC-Al30.