ReviewConstructed wetlands for boron removal: A review
Introduction
The elevating cost of energy in recent years together with operation cost of wastewater treatment have led to a strong interest to find alternative treatment strategies to conventional technologies (Tu et al., 2010). Constructed wetlands (CWs) represent an eco-friendly alternative for various types of wastewater around the world (Vymazal, 2009). Constructed wetlands are found in Europe, North America, South and Central America, Australia, New Zealand and Oceania, Africa and Asia (Vymazal and Kröpfelová, 2008). Probably more than 100,000 CWs worldwide currently treat over billion litres of water per day (Kadlec and Wallace, 2009, Zhi and Ji, 2012). With the recent rapid growth in wastewater treatment technologies, the issue of boron (B) treatment by CWs has come under the scientific spotlight. In recent years, several laboratory and field experiments have been carried out to determine how CW systems can be applied for B removal (Ye et al., 2003, Murray-Gulde et al., 2003, Kuyucak and Zimmer, 2004, Gross et al., 2007, Allende et al., 2012, Türker et al., 2013a, Türker et al., 2013b). However, the applicability of the CWs is not yet clearly assessed in view of recent findings on B removal.
In the current paper, B chemistry and behavior in aquatic environments, general B related problems in the aquatic environment and the current knowledge regarding the applicability of CWs to removal of B are reviewed. Boron removal mechanisms in sediment and the role of plants in CWs are also discussed in the review. Finally, data needed for full understanding of possible use of CWs for B removal are presented.
Section snippets
Chemical properties of boron
Boron is a metalloid (atomic weight 10.811, solid state 298 K, melting point 2349 K, boiling point 4200 K) in group 13 of periodic table, and it is not found as a free element in hydrosphere and lithosphere. It always binds with oxygen to form both borate minerals (borax, ulexite and colemanite) and orthoboric acid (Gemici et al., 2008, Wolska and Bryjak, 2013). The average B concentration in earth's crust is 10 mg kg−1 and varies from 1 to 500 mg kg−1 depending on the composition of substrate type (
Boron related problems in the environment
Soil environment is sensitive to pollutants because many bacterial activities and plants uptake are facilitated by dissolved phases of pollutants in the soil. Boron concentration in the soils varies between 2 and 100 mg kg−1, with an average concentration of 30 mg kg−1 (Nable et al., 1997). The high concentrations of B in the soil may be the source of B toxicity effects observed in plants (Camacho-Cristóbal et al., 2008). In the recent years a significant increase in the amount of B in soil has led
Boron in water
Boron is found as several species in water depending on their concentrations in water. The main factor that control B speciation is pH (Tu et al., 2010). At higher concentrations at high pH (≤10), polynuclear B species such as [B3O3(OH)5]2− and [B4O5(OH)4]2− are found in the water. However, at low concentrations, mononuclear species boric acid [B(OH)3], borate ions [B(OH)4−] or boric oxide (B2O3) would be dominant (Hilal et al., 2011). These B species are water soluble and as such are found in
Boron removal technologies
At present, there is no treatment technology which would be specifically designed for B removal. Therefore B is always removed in conjuction with the other target parameters. The treatment methods used for the removal of B include coagulation–electrocoagulation, adsorbsion on oxides, adsorbsion on active carbon, ion exchance with basic exhangers, electrodialysis, membrane filtration after complexation, use of B selective resins, with diols as B complexing agents (Table 1). The removal of B
Boron removal in constructed wetlands
Until now, many authors have documented efficient removal of various contaminants including metals or metalloids in CWs (Vymazal and Kröpfelová, 2008, Kadlec and Wallace, 2009, Marchand et al., 2010, Wu et al., 2013, Vymazal, 2013). The information on the removal of B in various types of CWs is limited (Table 2). Also, the removal processes in CWs which are responsible for B removal have not been understood clearly. The chemistry of B differs from that of other trace elements, and the overall B
Conclusions
Boron has special chemistry and, thus it mostly differs from that of other trace elements. Boron chemistry depends strongly on pH and ionic strength, and this behavior could be important parameter in B removal process in CWs. The literature on B removal in CWs is very limited, but several studies have showed that CWs have considerable potential to remove B from wastewaters. Nevertheless, there are some limitations to removal of B in CWs: time-consuming process, impact of climatic conditions,
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