Recent advances in membrane materials and technologies for boron removal

Recent advances in membrane materials and technologies for boron removal

Boron is not only an essential micronutrient for living beings, but also an important raw material for numerous industries, e.g. the pro- duction of fiberglass, detergents, fertilizers, etc. [1]. However, boron has been recently reinstated as an inevitable contaminant in various water supplies mainly for two reasons. Firstly, boron production and consumption present a steady growth in recent years, driven by the strong demand in the agricultural, ceramic, and glass markets [2]. The leakage of boron compounds to various waterbodies causes serious environmental issues. Secondly, boron toxicities to plants, animals and humans have been gradually unveiled by many researchers [2–4]. Thus， the boron concentration in drinking water has been regulated in most countries  and  regions, for example,  1.0 mg L−1  recommended  by  EU[5] and 2.4 mg L−1  by  WHO  [6]. In certain  industries  such  as  the semiconductor manufacturing sector, the control of boron in ultra-pure water is much more stringent [1,12]. Therefore, it is of paramount importance to remove boron from waterbodies in order to produce eligible water for various uses.

Membrane technologies for boron removal have received attention for few decades [7]. Nowadays, membrane-based separation processes have gained important roles as alternatives to conventional techniques in a broad spectrum of applications owing to their environmentally benignancy and efficiency in energy/cost consumption [8]. These membrane separation processes can be categorized into microfiltration， ultrafiltration, nanofiltration, reverse osmosis, electrodialysis, gas se- paration, pervaporation, membrane distillation, etc. based on the membrane properties and their mass transport mechanisms [9]. Efforts have been made to demonstrate the feasibility of several membrane processes for separation of boron from water. Examples are reverse osmosis (RO), forward osmosis (FO), nanofiltration (NF), ultrafiltration (UF) and membrane distillation (MD). In view of the booming trend of membrane applications in boron removal, an overview of the past de- velopment in membrane technologies is needed. Thus, this review aims to present a comprehensive summary of the recent advances in research and development of membrane technologies from the perspectives of membrane materials, membrane fabrication and system design. We believe that the information presented will provide useful insights about the membrane material and process design for deboronation in various aqueous systems.

2.Boron in the environment
As an element ubiquitous in nature, boron is extensively distributed in (1) lithosphere as borate minerals (e.g. Na2B4O7·10H2O, etc.) and (2) various water bodies primarily in the form of boric acid. The average boron  content  varies  from  1 to 500 mg kg−1 in the Earth's crust and 2–100mg kg−1 in soils [1]. In contrast, the main boron sources are the oceans in which the boron concentration ranges from 0.5 to 9.6 mg L−1 [10]. Boron content in uncontaminated surface and ground water is usually lower than 0.5 ppm. However, it has been found to be increased significantly in recent years as a result of anthropogenic activities [10–12]. The  boron  concentration  in  surface  water  can  go  up  to 100 ppm due to the boron waste discharge. For example, the back- ground boron concentration in one river stream in Liaoning province, China, was less than 0.5 ppm, while the boron concentrations of surface water and ground water in polluted reaches were 23.1 and 495.6 times of that in background areas, respectively [13]. This is probably because of the nearby mining operations. The potential of arousing environ- mental and health problems has raised tremendous concerns world- wide.

3.Boron toxicity
In spite of an inevitable micronutrient, boron can be toxic to living beings. Especially, plants are particularly sensitive to boron con- centration. For example,  it should be maintained  between  0.75 and 1.0 ppm in soil water for the good growth of wheat [14]. A higher boron dosage may slowly develop toxicity symptoms. The physiological leaf chlorophyll, inhibition of photosynthesis, deposition of lignin and suberin, increase in membrane leakiness, peroxidation of lipids and altered activities of anti-oxidation pathways, etc [3]. It should be noted that the boron toxicity is a function of both its concentration in soil water and the exposure duration. On the other hand, the adverse effects of boron to animals and humans have long been demonstrated in la- boratory animals. Long-term consumption of boron-contaminated water and food probably lead to syndromes and diseases in cardiac- vascular, nervous, and alimentary systems [2]. It was also reported by Ku et al. that short- and long-term oral exposures to boric acid or borax may cause negative impact on the male reproductive tract in rats [4]. Therefore, it is very important to control boron content in water sources and produce low boron containing water for various use, e.g. drinking and irrigation.

4.Boron chemistry

A basic understanding of boron chemistry is essential to the devel- opment of deboronation methods. Being the only non-metallic element in group 13 of the periodic table, boron has a quantum representation of 1s22s22p1. It is electron-scarce as a full valence shell with eight electrons cannot be completed by three single bonds. Thus, all boron is essentially in the trivalent oxidation state. In nature, boron is found in the form of boric acid, borate, or borosilicate mineral. Boric acid is soluble in water with a solubility of 5.5 g per 100 g solution at 25 °C. In aqueous solutions, it behaves as a Lewis acid. Its dissociation is achieved by accepting a hydroxyl ion to form the tetrahydroxyborate

The intrinsic pKa of boric acid is 9.24 under the standard condition, which  is in dilute  aqueous  solutions  at 25 ℃. The apparent  pKa  value depends on the external conditions, e.g. ionic strength and temperature. Dickson developed an empirical equation to correlate the dissociation of boric acid with salinity and temperature [15]. Based on his equation, the apparent pKa was plotted in Fig. 1(a) and (b) as a function of temperature and salinity, respectively. The negative correlations with temperature and salinity were also evidenced by other literatures. For example, it was reported that pKa decreases to 8.76 at 10℃ and 8.47 at 35 ℃ in a saline solution with a salinity of 3.5% [16].
The speciation of boric acid also depends on its concentration as shown in Fig. 1(c) and (d) [19]. The monomeric B(OH)3 or B(OH) − dominates at low boron concentrations (< 20 mM or 220 ppm as B) while polyborate species prevail at concentrations higher than 20 mm. It has been unveiled that the B3O3(OH) − species can be found in the concentration range of 20–200 mm [17], while additional species, B4O5(OH) 2−and BO(OH)−, may be present at elevated concentra-tions as shown in Fig. 2 [18,134]. Fig. 1(c) and (d) shows the fraction diagram of aqueous boron species as a function of pH at concentrations of 10 and 400 mm with a salinity of 3.5% and a temperature of 25℃ [17–19].

Another unique feature of boric acid is that it can form borate esters with poly-hydroxylated organics in an aqueous environment. Fig. 3 shows the general scheme of complexation reaction between boric acid and diols. The equilibrium constants of some diols and polyols are summarized in Table 1 [20]. In order to form a stable complex, the multi-hydroxyl groups must orient properly to match the structural parameters of the tetrahedrally coordinated boron [10,21]. For ex- ample, D-mannitol and D-sorbitol with two hydroxyl groups occupying vicinal cis positions have been demonstrated to form very stable borate complexes with high reaction constants [22]. The formation of mono- cyclic or dicyclic compounds with the polyol groups is also affected by the solution acidity as it generates hydrogen cations during the com- plexation. The hydroxide anion in the alkaline environment can neu- tralize the hydrogen cation generated during the complexation, which

The crystal ionic radius of boric acid is in the range 0.244–0.261 nm[23]. Due to its poor hydration capacity, the boric acid molecule in aqueous solutions is expected to have a similar size to the water mo- lecule.