Understanding salt and contaminant removal by a pervaporative treatment process
Creators: name: Sule, May Nyabochi
Publisher: name: Imperial College London
Year: 2013

description: This thesis summarises research into the performance of tubular, hydrophilic pervaporative polymer membranes in the removal of salt and organic contaminants from water, with an emphasis on organic micro-pollutants which are commonly found in oilfield-produced water. The treatment and recovery of oilfield-produced water for beneficial uses, such as agricultural irrigation, was a motivation for testing whether a sub-surface pervaporative irrigation process may be suitable for such applications. Previously there was very limited information regarding the removal of salt and organics from water by pervaporative membranes and only theoretical hypotheses regarding the relative removal of organics based on their chemical properties; this research expanded the knowledge in this area. The removal of salts, benzene, toluene, ethylbenzene, xylene (BTEX), humic acids (as a model for dissolved organic matter, DOM), fluorene, naphthalene, phenol, 1,2-diethylbenzene, 2-phenoxyethanol and 1,2-dichlorobenzene from water by pervaporation were considered experimentally. A solution-diffusion model was used to estimate the permeate (water) flux, selectivity and removal of contaminants. The results showed that the medium surrounding the tubular membrane, the contaminant type and concentration, and the membrane thickness were all factors which influenced the water flux as well as the selectivity of the membrane during the pervaporative treatment process. Sodium chloride rejection was generally very good, with &gt;98% (by mass) rejection at room temperature observed when starting with salt concentrations simulating seawater or highly saline oilfield-produced water (150,000 mg/l). Scanning electron microscopy images of the polymer showed that salt crystals occupy voids in the polymer layers within the membrane walls and that the membrane has a thin outer active layer that is responsible for most of the separation of salt from the water as the latter passes across the membrane. However, salt breakthrough occurred in cases when both sides of the membrane were in contact with liquid water. Hydrogen bonding and other chemical properties have an effect on the selectivity of organics removal. Phenol and 2-phenoxyethanol exhibited the lowest removals of 47% and 58% (by mass), respectively, while fluorene was best removed at 86%. Greater than 99% of BTEX compounds, of particular relevance to produced waters, were removed. Molecular weight, molecular volume, kinetic diameter and hydrogen bonding characteristics correlated well with removal (r &gt; 0.9), indicating that both a molecular sieving effect and hydrogen bonding are factors that determine the separation of organic contaminants from water by this pervaporative treatment process. Overall, this research confirmed previous findings that pervaporative membranes appear to be able to produce water fluxes which may be suitable for irrigation, however the rejection of salt and other contaminants was incomplete and contaminant penetration into soils should be closely monitored over the long term in irrigation trials, especially in cases where the surrounding soil becomes overly saturated with water.