Elsevier

Ecological Engineering

Volume 64, March 2014, Pages 350-359
Ecological Engineering

Review
Constructed wetlands for boron removal: A review

https://doi.org/10.1016/j.ecoleng.2014.01.007Get rights and content

Highlights

  • We reviewed current knowledge regarding to the CWs for boron (B) removal.

  • We presented general information about B, related problems in ecological components.

  • B removal mechanism in sediment and the role of plants in CWs were also discussed in detail and updated.

  • We identified data need that would contribute our understanding of possible use of CWs for B removal.

Abstract

Boron (B) contamination in the environment still increases because of various natural sources and anthropogenic activities. Therefore, the problem of removing B from water becomes a worldwide concern due to its toxicity and chronic effects on plants, animals and human health. This situation has generated increasing interest in the use of several wastewater treatment technologies in order to remove B from contaminated water. Constructed wetlands (CWs) present friendly alternative methods to treat wastewater around the world, and are used for removing various contaminants including metals and metalloids. This paper reviews current knowledge regarding the removal process of B, discusses application of B removal, and identifies critical knowledge study fields of future and gaps. Despite the fact that the sediment is a major sink for the removal of B, plants can play a significant role under favorable environmental conditions. The most important environmental factors that affect B removal in CWs are climatic conditions (e.g. transpiration rates), pH, temperature, solutions composition and competing species, hydraulic retention time and supporting media. Further research is needed on the major removal mechanism of B in CWs, namely the applicability of surface flow system, hybrid systems and vertical flow systems to remove B from wastewaters, the role of microorganism in order to enhance B removal efficiency.

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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