Performance and autopsy of nanofiltration membranes at an oil-field wastewater desalination plant

The mechanism of silica and transparent exopolymer particles (TEP) on reverse osmosis membranes fouling

Dissolved silica and transparent exopolymer particles (TEP) are the primary foulants in reverse osmosis (RO) desalinated brackish water and wastewater. In this study, we investigated the fouling properties of varying silica concentrations with TEP on the membrane surface and discovered a synergistic fouling effect between the silanol group (Si-OH) and the TEP carboxyl group (-COOH). The membrane fouling experiments showed that silica fouling approached saturation at 6 mM, with little variation in membrane flux as the silica concentration increased. Furthermore, the -OH functional group of the monosilicate molecule can chemically react with the -COO- functional group on the membrane surface to create hydrogen bonds, allowing monosilicate deposition directly on the membrane. Silica-silica interactions reacted with aggregates at high silica concentrations and joined with TEP to create a more substantial, more complex cross-linked network, resulting in severe membrane fouling. At pH 9, silica fouling was most severe due to the dramatic increase in the solubility of monosilicic acid dissolution in solution and the decreased polymerization rate. This work reveals the essential process of membrane fouling induced by silica and TEP, significantly increasing knowledge on silica-TEP fouling.

Climate change influence on the levels and trends of persistent organic pollutants (POPs) and chemicals of emerging Arctic concern (CEACs) in the Arctic physical environment - a review

Climate change brings about significant changes in the physical environment in the Arctic. Increasing temperatures, sea ice retreat, slumping permafrost, changing sea ice regimes, glacial loss and changes in precipitation patterns can all affect how contaminants distribute within the Arctic environment and subsequently impact the Arctic ecosystems. In this review, we summarized observed evidence of the influence of climate change on contaminant circulation and transport among various Arctic environment media, including air, ice, snow, permafrost, fresh water and the marine environment. We have also drawn on parallel examples observed in Antarctica and the Tibetan Plateau, to broaden the discussion on how climate change may influence contaminant fate in similar cold-climate ecosystems. Significant knowledge gaps on indirect effects of climate change on contaminants in the Arctic environment, including those of extreme weather events, increase in forests fires, and enhanced human activities leading to new local contaminant emissions, have been identified. Enhanced mobilization of contaminants to marine and freshwater ecosystems has been observed as a result of climate change, but better linkages need to be made between these observed effects with subsequent exposure and accumulation of contaminants in biota. Emerging issues include those of Arctic contamination by microplastics and higher molecular weight halogenated natural products (hHNPs) and the implications of such contamination in a changing Arctic environment is explored.

Interaction between humic acid and silica in reverse osmosis membrane fouling process: A spectroscopic and molecular dynamics insight

Combined organic and inorganic fouling is a primary barrier constraining the performance of reverse osmosis (RO) membrane. In this work, we conducted a systematic study focusing on the synergetic fouling effects of silica and humic acid (HA) in RO process, and found the critical silica concentration where the fouling pattern changed qualitatively. When the silica concentration was lower than 6 mM at a typical HA concentration of 50 mg·L^-1, no severe fouling was observed, while silica reaching this critical point could cause severe synergetic fouling with HA. Concentrated silica above the critical point acted as the prior foulant with marginal fouling effect caused by HA. A variety of solutions and surface-based characterizations were performed to elucidate the synergistic fouling responsibility for silica and HA. Our study suggests that the carboxylic groups from HA formed hydrogen bonds with silica hydrate, inducing silica adsorption onto HA aggregates at low silica particle concentrations. The HA network was bridged together to form large foulants due to the silica-silica interaction above the silica critical concentration. These mechanisms were further confirmed by molecular dynamics simulations. This study provides an in-depth insight into the combined organic-inorganic fouling and can serve as a guideline to optimize feed conditions in order to mitigate fouling of RO in wastewater reusing industry.

Intensification of the ultrafiltration of real oil-contaminated (produced) water with pre-ozonation and/or with TiO2, TiO2/CNT nanomaterial-coated membrane surfaces

In the present study, commercial PES, PVDF, PTFE ultrafilter membranes, and two different nanomaterial (TiO₂ and TiO₂/CNT composite)-covered PVDF ultrafilter membranes (MWCO = 100 kDa) were used for the purification of an industrial oil-contaminated (produced) wastewater, with and without ozone pretreatment to compare the achievable fouling mitigations by the mentioned surface modifications and/or pre-ozonation. Fluxes, filtration resistances, foulings, and purification efficiencies were compared in detail. Pre-ozonation was able to reduce the total filtration resistance in all cases (up to 50%), independently from the membrane material. During the application of nanomaterial-modified membranes were by far the lowest filtration resistances measured, and in these cases, pre-ozonation resulted in a slight further reduction (11-13%) of the total filtration resistance. The oil removal efficiency was 83-91% in the case of commercial membranes and > 98% in the case of modified membranes. Moreover, the highest fluxes (301-362 L m^-2 h^-1) were also measured in the case of modified membranes. Overall, the utilization of nanomaterial-modified membranes was more beneficial than pre-ozonation, but with the combination of these methods, slightly higher fluxes, lower filtration resistances, and better antifouling properties were achieved; however, pre-ozonation slightly decreased the oil removal efficiency.