Elsevier

Journal of Membrane Science

Volume 549, 1 March 2018, Pages 456-465
Journal of Membrane Science

The roles of pH and draw solute on forward osmosis process treating aqueous naphthenic acids

https://doi.org/10.1016/j.memsci.2017.12.029Get rights and content

Highlights

  • The effect of pH and draw solutes on the rejection of NA model compounds was studied.

  • From pH 3 to 9, electrostatic repulsion mechanism dominated CHA and AAA rejection.

  • Merichem NA rejection was due to its larger size and electrostatic repulsion forces.

  • Membrane surface contact angle changed with the NA properties in the feed solution.

  • CaCl2 as draw solution led to a more rapid flux decline.

Abstract

The effects of pH and draw solutes on the rejection of three naphthenic acid (NA) model compounds, namely cyclohexane carboxylic acid (CHA), 1-adamantaneacetic acid (AAA) and the refined Merichem mixture of NAs, by forward osmosis (FO) were studied. The rejection behavior of CHA and AAA were pH-depended (from pH 3 to 9) which further suggested that electrostatic repulsion was the dominant rejection mechanism. The rejection efficiency of Merichem NAs was maintained above 95%, which was not affected by the pH change (from 6 to 9). The water flux decline when Merichem NAs was used as feed solution was partially attributed to membrane surface fouling. Membrane surface contact angle changed with the properties of NAs in feed solution, which might suggest that the exposure of NA model compounds altered the membrane characteristics. Four inorganic salts—sodium chloride (NaCl), ammonia chloride (NH4Cl), sodium sulphate (Na2SO4), and calcium chloride (CaCl2)—were introduced as the draw salts and no significant difference was found between these draw solutes regarding the CHA rejection at pH = 9. The decreased reverse salt flux along with the water flux decline indicated that using CaCl2 as the draw solution caused membrane surface precipitation.

Introduction

The Clark hot water extraction method is widely applied by the Canadian oil sands industry, in which large amounts of water are used to separate the bitumen from the oil sands. As a result, both organic and inorganic compounds are dissolved into the process water creating oil sands process-affected water (OSPW). The organic content of OSPW is made up of naphthenic acids (NAs), polycyclic aromatic hydrocarbons (PAHs), benzene, toluene, ethylbenzene, xylene (BTEX) and phenols [1], [2]. Nearly 50% of the extractable organic fraction in OSPW is NAs which are considered to be one of the primary sources of OSPW toxicity towards aquatic species [3]. NAs have an empirical structure formula of CnH2n+ZO2, where n represents the number of carbon atoms and Z is the deficiency of hydrogen atoms [4]. Typically, the NAs found in OSPW have n number varying from 7 to 30 and the Z number ranging from −12 to 0 [3], [5]. The concentrations of these complex ringed compounds range from 40 to 120 mg/L in the various tailing ponds, depending on the mining process and the pond age [6]. Various treatment processes, including physical, chemical, and biological approaches have been conducted to explore the NA removal in OSPW. For membrane process, the reported rejection efficiency for NAs in OSPW varies from 12.4% to 95%, depending on the membrane types and pre-treatment methods [7], [8], [9].

Forward osmosis (FO) is a membrane-based process that employs osmotic pressure difference between draw and feed solution as driving force [10]. In FO process, ideally, only water molecules can be drawn from the less concentrated feed solution to the concentrated draw solution and those undesirable contaminants can be retained in the feed side. Along with high rejection efficiency, FO owns a few advantages over pressure-driven membrane process including less membrane fouling, reduced energy cost, and simplified operating system [10]. Therefore, applying FO to treat oil and gas wastewater is attracting more research interests [11], [12], [13]. FO has been incorporated in the reclamation of oil and gas drilling wastewater [14], [15], separation of emulsified oil–water [12], and recovery water from petroleum/water emulsions [16], among others. Desirable organic rejections and high water recovery can be achieved with the help of FO process in the treatment of oil and gas wastewater [13], [14].

Rejections of organic compounds can be affected by several factors including properties of target compounds, membrane characteristics, and feed/draw solution chemistry [17]. For instance, Jin et al. [18] investigated the rejection of pharmaceuticals including diclofenac, carbamazepine, ibuprofen, and naproxen by commercial cellulose triacetate (CTA) and thin film composite (TFC) FO membranes. The authors found that TFC exhibited a stable performance in terms of contaminant rejection under various pH conditions. They also suggested that, for CTA membranes, size and hydrophobicity of compounds were two important factors affecting the rejection of tested pharmaceuticals at low pH level. Moreover, Cui et al. [19] studied the removal of phenol, aniline, and nitrobenzene via laboratory fabricated FO membrane using feed/draw solutions with different concentration level. It was reported that the rejection efficiencies were barely affected by the rejection of organic micro-pollutant at concentration lower than 2000 ppm. The authors also observed that, increasing the draw solution concentration, in other words, increasing water flux, can benefit the organic contaminant removal. Besides the influence of pH values and membrane types, membrane fouling also needs to be mentioned. Linares et al. [20] evaluated the rejection of organic micro-pollutants using clean and fouled FO membranes. The experimental results illustrated that the hydrophilic ionic compounds showed a higher rejection in both clean and fouled membranes and the fouling layering improved the rejection of all the micro-pollutants due to the increased hydrophilicity and membrane surface charge.

In this study, the effects of feed solution pH and draw solutes were examined as two important factors affecting the NA rejection and OSPW filtration performance. The rejections of NA model compounds, including cyclohexane carboxylic acid (CHA), 1-adamantaneacetic acid (AAA) and the refined Merichem mixture of NAs, were evaluated at different pH conditions. Also, four draw solution (i.e., NaCl, NH4Cl, CaCl2 and Na2SO4) were studies in order to evaluate their influence on NA removal in terms of reverse salt diffusion and permeate water flux.

Section snippets

Bench-scale FO system

A schematic experimental set-up is shown in Fig. 1. The cellulose triacetate (CTA) membrane was purchased from HTI (Hydration Technologies Innovations, Inc., Albany, OR, USA). The commercial CTA membrane was fabricated asymmetrically with a smooth active layer and an embedded polyester mesh support. Detailed membrane characteristics were described elsewhere [21], [22], [23]. The FO membrane was placed in a SEPA FO membrane cell provided by Sterlitech Corporation (Kent, WA, USA) with the feed

Water flux

Fig. 2 presents the baseline corrected water flux in the rejection experiments at various pH values using CHA, AAA and Merichem NAs as feed solution and 1 M NaCl as draw solution, respectively. As a reference, water flux of pH adjusted OSPW (NA concentration = 25.4 mg/L) is also shown in Fig. 2(d). No significant changes on water flux were observed with respect of pH when applying CHA and AAA aqueous solution as the feed. Decreasing the initial CHA and AAA concentrations from 100 to 25 mg/L did

Conclusions

The current study presented the effect of pH and four draw solutes – NaCl, NH4Cl, Na2 SO4 and CaCl2 – on the rejection of selected NA model compounds including CHA, AAA and Merichem NAs in 7 h. The results indicated that from pH 3 to 9, electrostatic repulsion mechanism dominated CHA and AAA rejection. From pH 6 to 9, the rejection of Merichem NAs was stable (> 92%) likely due to its larger size and electrostatic repulsion force of membrane. At pH 9, the overall rejection of three NA model

Acknowledgements

This research was supported by a research grant from a Natural Sciences and Engineering Research Council of Canada (NSERC) Senior Industrial Research Chair (IRC) Program in Oil Sands Tailings Water Treatment through the support by Syncrude Canada Ltd., Suncor Energy Inc., Shell Canada, Canadian Natural Resources Ltd., Total E&P Canada Ltd., EPCOR Water Services, IOWC Technologies Inc., Alberta Innovates – Energy and Environment Solution, and Alberta Environment and Parks. The financial supports

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