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Arsenic Poisoning through Ages
Published in M. Manzurul Hassan, Arsenic in Groundwater, 2018
The trivalent form of arsenic species is the dominant species under reducing conditions, and the pentavalent form of arsenic species is the dominant species under oxidizing conditions (Swami et al., 2014). Depending on the redox condition, pH range, presence of complexing ions and the microbial activity of the environment, the pentavalent form of arsenic species may exist as arsenic acid (HaAsO4) and arsenate ions (AsO43−). Similarly, the trivalent form of arsenic species may be present as arsenious acid (H3AsO3) and arsenite ions (AsO33−) (Swami et al., 2014). Pentavalent arsenicals (arsenate) are more likely to occur in aerobic surface waters and trivalent arsenicals (arsenite) are more likely to occur in anaerobic ground waters (USEPA, 2000).
Arsenic
Published in Pankaj Chowdhary, Abhay Raj, Contaminants and Clean Technologies, 2020
Kiran Gupta, Alka Srivastava, Amit Kumar
Arsenic has various oxidation states, regardless of whether it is present in organic or inorganic compounds (Tareq et al., 2003). The mobility of As existing in the inorganic compound in contaminated water is determined by precipitation, redox processes, sorption, and dissolution. In oxic groundwater, the role of ferric iron phase is highly significant for the sorption of dissolved arsenate (Zobrist et al., 1998; Root et al., 2009). Generally, As occurs in four oxidation states, viz., As−3 (arsine), As0 (arsenic), As+3 (arsenite), and As+5 (arsenate) (Oremland and Stolz, 2005). Smedley and Kinniburgh (2002) depicted that in the most cases of inflated groundwater As concentration, the aquifer sediments of the same contain about 1–20 mg kg−1 As although rather than being fortified with As. On a regional scale, a high concentration of As needs a geochemical trigger, which releases As from a solid state to groundwater and also which allows As to prevail in solution in groundwater. Thermodynamically, As+3 is infirm and can be converted to As+5 via oxidation in aerobic conditions. However, the process of oxidation occurs very slowly when only oxygen is present as an oxidant. Oxides of Fe and Mn increase the oxidation rate. As in groundwater is mostly determined by adsorption. This process is a complex function of interdependencies among pH, As and competing ion concentration, solid surface properties, and speciation of As. Oxides of Al, Mn, and Fe are usually the most significant originator or dig of arsenic because of their universal occurrence, chemical properties, and having an approach to coat else particles. Adsorption of arsenite and arsenate is pH dependent. Arsenate adsorption occurs very efficiently at lower pH but it decreased beyond pH 7. In spite of this, arsenite adsorption is directly proportional to pH, and the maximum adsorption has been reported at pH 8–9. In typical groundwater conditions, As occurs either in As3+ (reduced arsenite) state or in the As5+ (oxidized arsenate) state. Arsenic ions associate with water to construct many aqueous species. For example, As3+ ion reacts with three water molecules to form arsenious acid (H3AsO3) (10.1): As3++3H2O=H3AsO3+3H+
Parametric study of adsorption column for arsenic removal on the basis of numerical simulations
Published in Waves in Random and Complex Media, 2022
Sikandar Almani, Khadija Qureshi, Kashif Ali Abro, Masroor Abro, Imran Nazir Unar
In this study, water containing arsenic is lumped into single specie called arsenious acid (H3AsO3). The arsenious acid is considered as a mobile phase in the adsorption column. Whereas, iron oxide (FeOH) is used as an adsorbent in the form of stationary active sites in the shape of small circular balls. The arsenious acid has been introduced from the top of the column with a mass flow rate. As the arsenious acid flows through the active sites, the surface reaction occurs between arsenious acid and iron oxide as mentioned by Equation (1) [15]. While as the reaction proceeds, the iron oxide (FeOH) active sites adsorb arsenic and pure water is withdrawn at the bottom of the adsorption column. The chemical reaction is as follows:
The Fate of the Arsenic Species in the Pressure Oxidation of Refractory Gold Ores: Practical and Modelling Aspects
Published in Mineral Processing and Extractive Metallurgy Review, 2023
Wei Sung Ng, Yanhua Liu, Qiankun Wang, Miao Chen
Dissolved arsenic in the autoclave discharge is present in either trivalent form as arsenous acid (H3AsO3) or pentavalent form as arsenic acid (H3AsO4). While the pentavalent state is more likely to be the common form following pressure oxidation, a portion may be present as arsenous acid due to incomplete oxidation into arsenic acid (Brookins 2012; Sadiq et al. 2002; Smedley and Kinniburgh 2002). The pentavalent form is important for the formation of stable arsenate phases, as it is more amenable to (co)precipitation in the presence of metal cations (Ritcey 2005), in comparison with the trivalent form, while being less toxic and less mobile at low pH values. Interestingly, it has been observed that the trivalent form may be dominant at the elevated temperatures found during pressure oxidation, returning to the pentavalent form at reduced temperatures (Chen et al. 2018). This has poor implications for the precipitation of arsenate during pressure oxidation, suggesting that a downstream curing stage might be necessary to provide an environment with higher concentrations of As(V) and to encourage the formation of mineral arsenates. Dissociation and deprotonating of the acids are not expected due to the acidity of the autoclave mixture. Where acid recycling is implemented, the dissolved species represents a source of arsenic in the autoclave feed. Dissolved arsenic that is not precipitated will need to be treated, typically by neutralization and precipitation with lime. This is important to ensure that arsenic levels are below regulatory limits prior to disposal and to prevent drainage and escape of the arsenic.
The roles of membrane transporters in arsenic uptake, translocation and detoxification in plants
Published in Critical Reviews in Environmental Science and Technology, 2021
Arsenite [As(III)] has a high pKa (9.2) and is therefore present predominantly as undissociated neutral molecule (arsenous acid) in physiologically relevant pH range (<7.5) (Zhao, Ago et al., 2010). This physicochemical property also explains why As(III) is transported across membranes by some members of the subfamilies of plant major intrinsic protein (MIP) superfamily, such as nodulin 26-like intrinsic proteins (NIP), plasma membrane intrinsic proteins (PIP) and tonoplast intrinsic proteins (TIPs) (Bienert et al., 2008; Bienert & Jahn, 2010; Mosa et al., 2012). The MIP family proteins, also termed as aquaporins, comprises channels for the diffusion of small neutral and mostly polar molecules across various biological membranes in all kingdoms of life. Genes encoding some members of NIPs are prevalently present in the arsenic-resistance-operons of various prokaryotes, and the encoded proteins are thought to be the most ancient transporters involved in As(III) resistance (Pommerrenig et al., 2020). Plant NIPs might have evolved from bacterial As(III) efflux channels. There are four phylogenetically supported prokaryotic MIP clades, including aquaporin Z‐like proteins (AqpZ), aquaporin M‐like proteins (AqpM), glycerol uptake facilitator‐like proteins (GlpF), and aquaporin N‐like proteins (AqpN) (Pommerrenig et al., 2020). It has been suggested that horizontal gene transfer of the bacterial AqpN group was the starting‐point for the evolution of plant NIPs (Pommerrenig et al., 2020). NIPs with the ancestral bacterial AqpN selectivity filter composition consisting of FR1–AR2–AR3–RR4 residues exist prevalently in algae, liverworts, moss, hornworts and ferns. These archetypic plant NIPs and their prokaryotic progenitors are almost impermeable to water and silicic acid (Si) but can transport As(III) and boric acid (Pommerrenig et al., 2020). During evolution, ancestral NIP selectivity might have shifted to allow water and silicon transport in seed plants (Pommerrenig et al., 2020).