Barbara Casentini

Anaerobic arsenite oxidation with an electrode serving as the sole electron acceptor: A novel approach to the bioremediation of arsenic-polluted groundwater

Arsenic contamination of soil and groundwater is a serious problem worldwide. Here we show that anaerobic oxidation of As(III) to As(V), a form which is more extensively and stably adsorbed onto metal-oxides, can be achieved by using a polarized (+497 mV vs. SHE) graphite anode serving as terminal electron acceptor in the microbial metabolism. The characterization of the microbial populations at the electrode, by using in situ detection methods, revealed the predominance of gammaproteobacteria. In principle, the proposed bioelectrochemical oxidation process would make it possible to provide As(III)-oxidizing microorganisms with a virtually unlimited, low-cost and low-maintenance electron acceptor as well as with a physical support for microbial attachment.

Arsenic removal from naturally contaminated waters: a review of methods combining chemical and biological treatments

The presence of arsenic (As) in aquatic environments is often attributable to geogenic processes occurring within aquifers. Arsenic exists in two prevalent oxidation states, with the trivalent arsenite [As(III)] exerting stronger toxicity effects on the aquatic biota than the pentavalent arsenate [As(V)]. The review evaluates the literature available on the main arsenic removal technologies and the application of combined chemical and biological treatments. We provide a synthetic outlook on the potential strategies of biological As(III) oxidation to As(V) by means of cell-detoxifying mechanisms or metabolic processes with the aim to enhance As removal efficiency. Furthermore, the role of microorganisms in the mobility of arsenic in natural systems as well as the distribution of As-resistant bacteria, potentially suitable for arsenic removal, is discussed in the context of a case study carried out in Latium region (Italy), which is known for arsenic contamination of waters.

Arsenic-related microorganisms in groundwater: a review on distribution, metabolic activities and potential use in arsenic removal processes

Groundwater plays a central role in the hydrological cycle and represents the utmost natural resource for human consumption and activities on a global scale. Therefore, any source of contamination of either geogenic or anthropogenic origin may provide a serious environmental health threat. Within the long list of organic and inorganic groundwater contaminants, arsenic, a toxic element retrieved in air, soils, rocks, waters and organisms, can occur at high concentrations in aquifers representing an issue of worldwide concern. Over the past years, research efforts aimed to elucidate the microorganisms and mechanisms involved in the biogeochemical cycling of this element. An emerging challenge is to identify and exploit microbial metabolic potentialities for arsenic-contaminated water treatment. The objective of this review is to outline the existing knowledge about ecology, biochemistry and genomics of arsenic-related microorganisms, with particular reference to their distribution and their capabilities to oxidize As(III) in groundwater. Moreover, a broad evaluation of the application potentialities of microbiological processes suitable for treatment strategies of arsenic-contaminated groundwater is provided.

Phylogenetic Structure and Metabolic Properties of Microbial Communities in Arsenic-Rich Waters of Geothermal Origin

Arsenic (As) is a toxic element released in aquatic environments by geogenic processes or anthropic activities. To counteract its toxicity, several microorganisms have developed mechanisms to tolerate and utilize it for respiratory metabolism. However, still little is known about identity and physiological properties of microorganisms exposed to natural high levels of As and the role they play in As transformation and mobilization processes. This work aims to explore the phylogenetic composition and functional properties of aquatic microbial communities in As-rich freshwater environments of geothermal origin and to elucidate the key microbial functional groups that directly or indirectly may influence As-transformations across a natural range of geogenic arsenic contamination. Distinct bacterial communities in terms of composition and metabolisms were found. Members of Proteobacteria, affiliated to Alpha- and Betaproteobacteria were mainly retrieved in groundwaters and surface waters, whereas Gammaproteobacteria were the main component in thermal waters. Most of the OTUs from thermal waters were only distantly related to 16S rRNA gene sequences of known taxa, indicating the occurrence of bacterial biodiversity so far unexplored. Nitrate and sulfate reduction and heterotrophic As(III)-oxidization were found as main metabolic traits of the microbial cultivable fraction in such environments. No growth of autotrophic As(III)-oxidizers, autotrophic and heterotrophic As(V)-reducers, Fe-reducers and oxidizers, Mn-reducers and sulfide oxidizers was observed. The ars genes, involved in As(V) detoxifying reduction, were found in all samples whereas aioA [As(III) oxidase] and arrA genes [As(V) respiratory reductase] were not found. Overall, we found that As detoxification processes prevailed over As metabolic processes, concomitantly with the intriguing occurrence of novel thermophiles able to tolerate high levels of As.

Biological As(III) oxidation in biofilters by using native groundwater microorganisms

Arsenic (As) contamination in drinking water represents a worldwide threat to human health. During last decades, the exploitation of microbial As-transformations has been proposed for bioremediation applications. Among biological methods for As-contaminated water treatment, microbial As(III)-oxidation is one of the most promising approaches since it can be coupled to commonly used adsorption removal technologies, without requiring the addition of chemicals and producing toxic by-products. Despite the As(III) oxidation capability has been described in several bacterial pure or enrichment cultures, very little is known about the real potentialities of this process when mixed microbial communities, naturally occurring in As contaminated waters, are used. This study highlighted the contribution of native groundwater bacteria to As(III)-oxidation in biofilters, under conditions suitable for a household-scale treatment system. This work elucidated the influence of a variety of experimental conditions (i.e., various filling materials, flow rates, As(III) inflow concentration, As(III):As(V) ratio, filter volumes) on the microbially-mediated As(III)-oxidation process in terms of oxidation efficiency and rate. The highest oxidation efficiencies (up to 90% in 3 h) were found on coarse sand biofilters treating total initial As concentration of 100 μg L-1. The detailed microbial characterization of the As(III) oxidizing biofilms revealed the occurrence of several OTUs affiliated with families known to oxidize As(III) (e.g., Burkholderiaceae, Comamonadaceae, Rhodobacteraceae, Xanthomonadaceae). Furthermore, As-related functional genes increased in biofilter systems in line with the observed oxidative performances.