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Direct contact membrane distillation (DCMD) has immense potential in the desalination of highly saline wastewaters where reverse osmosis is not feasible. This study evaluated the potential of DCMD for treatment of produced water generated during extraction of natural gas from unconventional (shale) reservoirs. Exhaust stream from Natural Gas Compressor Station (NG CS), which has been identified as a potential waste heat source, can be used to operate DCMD thereby providing economically viable option to treat high salinity produced water. An ASPEN Plus simulation of DCMD for the desalination of produced/saline water was developed in this study and calibrated using laboratory-scale experiments. This model was used to optimize the design and operation of large scale systems and estimate energy requirements of the DCMD process. The concept of minimum temperature approach used in heat exchanger design was

A B S T R A C THydraulic fracturing used for natural gas extraction from unconventional onshore resources generates large quantities of produced water that needs to be managed efficiently and economically to ensure sustainable development of this industry. Membrane distillation can serve as a cost effective method to treat produced water due to its low energy requirements, especially if waste heat is utilized for its operation. This study evaluated the performance of commercially available hydrophobic microfiltration membranes in a direct contact membrane distillation system for treating very high salinity (i.e., up to 300,000 mg/L total dissolved solids) produced water. Polypropylene and polytetrafluoroethylene membranes yielded the highest permeate flux with membrane distillation coefficient of 5.6 l/m 2 /hr/kPa (LMH/kPa). All membranes showed excellent rejection of dissolved ions, including naturally occurring radioactive material (NORM), which is a significant environmental concern with this high salinity wastewater. Analysis of membranes after extended testing with actual produced waters revealed unevenly distributed inorganic deposits with significant iron content. A key finding of this study is that the iron oxide fouling layer had negligible effect on membrane performance over extended period of time despite its thickness of up to 12 µm. The results of this study highlight the potential for employing membrane distillation to treat high salinity wastewaters from unconventional gas extraction.

The impact of membrane cleaning with NaOH and HCl on the characteristics and associated changes in ion rejection was investigated in this study. NaOH affected the zeta potential of membranes with a greater concentration of carboxylic groups so that it was negative across the entire pH range investigated. Exposure to NaOH led to swelling of the active layer after each cleaning, especially for poly­(piperazineamide) membranes. A 23% increase in the effective pore radii for these membranes after NaOH cleaning for 18 h led to 25, 36, 53 and 62% decrease in the rejection of magnesium, calcium, sodium, and chloride ions, respectively. Sulfate rejection decreased only slightly even for poly­(piperazineamide) membranes (i.e., 7%) despite an appreciable increase in pore radii, which can be explained by the impact of charge exclusion on ion rejection that was enhanced by the 16% reduction in zeta potential. On the other hand, cleaning with HCl had a negligible impact on the zeta potential and performance of all membranes evaluated in this study. The increase in permeability after chemical cleaning was in agreement with the decrease in rejection of inorganic ions and correlated well with the effective pore radii measured using the membrane potential technique. The importance of charge exclusion in the rejection of inorganic ions was highlighted by the observed differences in rejection and permeability values when testing membranes after NaOH cleaning.

Graphical AbstractThe increase in oxygen functionalities on GO with increasing use of oxidizing agent, results in (i) amplification of redox pseudocapacitive current and (ii) improves metal ion adsorption. Abstract 1Graphene oxide (GO) samples were prepared at room temperature using modified Hummer's 2 method. The quantitative variation of oxidizing agent for the oxidation of graphene sheets 3 resulted in increasing the oxygen functionalities on GO samples. The Qualitative analysis of 4 functional groups and surface charge variation was studied using Fourier transform infra-red 5 (FTIR) spectroscopy and zeta potential respectively. Different oxidation degrees of GO was 6 investigated by X-ray diffraction (XRD), Raman and X-ray photoelectron spectroscopy (XPS). 7The electrochemical charge storage property of the GO samples were studied using two 8 electrode supercapacitor cell. The fabricated supercapacitor demonstrates linear enhancement in 9 the specific charge storage with an increase in the oxidation of GO samples. Maximum charge 10 storage of 71 F/g has been obtained with highly oxidized GO sample at room temperature. The 11 adsorption of metal ions from the aqueous solution has also been studied with the variation in the 12 degree of functionalization of the GO samples. It was observed that increasing oxygen 13 functionalities from GO-1 to GO-5 amplifies the uptake of metal ions [Cd(II) and Cu(II)]. The 14 experimental data fits well in Langmuir adsorption model, indicating monolayer adsorption of 15 metal ion on GO samples.16