Removal of metals from contaminated waters : Evaluating the use of synthetic adsorbents and green algae biomass

Abstract: Metal contamination of water bodies has attracted global attention owing to its environmental toxicity, abundance and persistence. Metals such as arsenic, cadmium and mercury are of greater concern since they are toxic even at low concentrations. Mercury is present at 1000 times lower concentrations than the other two metals, but still has a negative impact on the environment due to its high transfer efficiency in the trophic chain. The major emissions of metals into the environment come from anthropogenic activities. Hence, in order to reduce the extent of metals mobilization in the environment, new water treatment technologies have to be suitable in providing high-quality treatments and remove low concentration of metals. In this thesis, different types of water contaminated with metals were treated by adsorption and bio-uptake processes. Two adsorbents were evaluated for treatment of arsenic and cadmium. In both cases, the adsorbents were composites composed of small-sized particles in the order of micrometer and nanometers, and a support material. The novel configurations, coated or embedded, not only improved the adsorption capacities of both materials, but also allowed the application in column or batch systems. The adsorbent evaluated for treating arsenic (macroporous polymer coated with co-precipitated iron aluminium hydroxides, named MHCMP) showed higher affinity for arsenic than a commercial one at low arsenic concentrations. In addition, the adsorbent for treating cadmium (silicate-titanate nanotubes embedded in chitosan beads, named STNTs-Ch beads) showed higher selectivity towards cadmium and lead over nontoxic metals when tested for treating biogas leachate. This adsorbent was also recycled thereby prolonging its life span. Furthermore, a life cycle assessment was carried out for the STNTs-Ch beads, which showed that electricity was the main input causing great impact on the environment and should be further optimized. The STNTs-Ch beads were also evaluated for treating synthetic municipal wastewater as a way to prevent toxic effects of metals on the bacteria in biological phosphorus removal treatment. The results were comparable to those obtained using a commercial ionic exchange resin. In contrast to adsorption, a biological material constituted of living green algae biomass showed high potential for uptake of methylmercury and could represent a remediation tool for a specific case of a high altitude contaminated lake in South America. The evaluated materials show high potential for the treatment of specific types of waters and further studies should focus on improving mechanical strength of support materials and proposing safe and sustainable scaling-up processes.