Aqueous contamination by oxyanions. The use of Zn-Al sulphate layered double hydroxides for waste treatment

ARDAU, CARLA;Davantes A;Dore E;FRAU, FRANCO;Lattanzi P.

2013-01-01

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Abstract

Sorption is regarded as one of the most promising technologies for the treatment of waste waters, due to its efficacy and cost-effectiveness. Layered double hydroxides (LDHs) is a class of compounds with general formula [M2+ 1- xM3+ x(OH)2](An-)x/n.mH2O. Possible cationic and anionic associations are widely variable (e.g. M2+ = Mg, Zn, Cu; M3+ = Al, Fe; An- = CO3 2-, NO3 -, SO4 2-). Thanks to their large surface area, and high anionic exchange capacity, LDHs are extensively investigated for their possible use as removers of pollutant oxyanions from waters. LDHs form spontaneously in a wide range of environments, including mine areas. Zn-Al-sulphate LDHs were found to precipitate from surface waters interacting with mine wastes [1]. Their effectiveness in attenuation of As content in NAMD (Net Alkaline Mine Drainage), observed in natural systems, was the starting point of our investigation. Synthetic Zn-Al-sulphate LDHs were reproduced in laboratory and tested in several batch sorption experiments, first for the uptake of As and, then, for less conventional environmental oxyanion pollutants (Mo, W and Sb). The first results in the use of these LDHs as removers of pollutant oxyanions (up to ~90% As(V); up to ~54% Mo; W and Sb work in progress) encourage further investigations. Since the affinity for LDHs strictly depends on the combination of anion charge density and size [2], a key point for sorption efficacy is the anionic form of the investigated pollutant (e.g. H2AsO4 -, HAsO4 2- or AsO4 3-), and of potential competitors in solution (e.g. HCO3 - and CO3 2-). For some of elements (Mo), possible polymerisation phenomena must also be considered (see also Davantès et al, this issue). Therefore, the optimal pH conditions must be specifically evaluated for any oxyanion and waste water composition in order to enhance the LDHs uptake efficacy. [1] Ardau et al, (2011). N. Jb. Miner. Abh. 188/1, 49–63. [2] Miyata S, (1983). Clays Clay Miner. 31 (4), 305–311.