Boron and sulfate removal using rice husk as filtration material in horizontal flow constructed wetlands microcosms

Constructed wetlands (CW) are being explored as an alternative treatment for boron removal. The efficiency of CW strongly depends on the substrate and plants used. A promising and inexpensive substrate is rice husk. This study explored the capacity of this residue to treat boron-rich wastewaters as main wetland substrate in laboratory constructed wetland microcosms. Two plants, Typha angustifolia and Puccinellia frigida, were used to evaluate their capacity to increase boron removal. It was found that CW removed ~40% of boron. Although both species presented high boron concentrations in their shoots (between 463 and 721 mg/kg), they did not enhance its removal. Interestingly, high sulfate removal was also observed, especially in non-vegetated cells (up to 68±18%), indicating that the presence of plants decreased their removal. Processes involved are probably biotic sulfate reduction and abiotic precipitation of sulfate minerals (hypothesis supported by PHREEQC modeling). Therefore, the performance of the system under continuous flow operation for a boron-and sulfate-rich wastewater indicates that rice husk has high potential as main media in CW for boron and sulfate removal