Arsenic immobilization

Bothe, J.V. Jr. and Brown, P.W. (1999) Arsenic immobilization by calcium arsenate formation. Environmental Science and Technology, 33(21), 3806-811. 

Ohnuki, T., Sakamoto, F., Kozai, N. et al. (2004) Mechanisms of arsenic immobilization in a biomat from mine discharge water. Chemical Geology, 212(3-4 SPEC.ISS.), 279-90. 

Moon, D. H., Dermatas, D. and Menounou, N. (2004). Arsenic immobilization by calcium-arsenic precipitates in lime treated soils. Sci. Total Environ. 330(1-3), 171-185. 

Arsenic is another element with different bioavailabiUty in its different redox states. Arsenic is not known to be an essential nutrient for eukaryotes, but arsenate (As(V)) and arsenite (As(III)) are toxic, with the latter being rather more so, at least to mammals. Nevertheless, some microorganisms grow at the expense of reducing arsenate to arsenite (81), while others are able to reduce these species to more reduced forms. In this case it is known that the element can be immobilized as an insoluble polymetallic sulfide by sulfate reducing bacteria, presumably adventitiously due to the production of hydrogen sulfide (82). Indeed many contaminant metal and metalloid ions can be immobilized as metal sulfides by sulfate reducing bacteria. 

Other special bombs were needed to blow up dams on the Dnieper River in Russia and thus immobilize 600,000 Germans. The Arsenal planned the bombs, made them and shipped them. They did the job... 

In addition to effects on the concentration of anions, the redox potential can affect the oxidation state and solubility of the metal ion directly. The most important examples of this are the dissolution of iron and manganese under reducing conditions. The oxidized forms of these elements (Fe(III) and Mn(IV)) form very insoluble oxides and hydroxides, while the reduced forms (Fe(II) and Mn(II)) are orders of magnitude more soluble (in the absence of S( — II)). The oxidation or reduction of the metals, which can occur fairly rapidly at oxic-anoxic interfaces, has an important "domino" effect on the distribution of many other metals in the system due to the importance of iron and manganese oxides in adsorption reactions. In an interesting example of this, it has been suggested that arsenate accumulates in the upper, oxidized layers of some sediments by diffusion of As(III), Fe(II), and Mn(II) from the deeper, reduced zones. In the aerobic zone, the cations are oxidized by oxygen, and precipitate. The solids can then oxidize, as As(III) to As(V), which is subsequently immobilized by sorption onto other Fe or Mn oxyhydroxide particles (Takamatsu et al, 1985). 

Precipitation is the most promising method for immobilizing dissolvable metals such as lead, cadmium, zinc, and iron.15 Some forms of arsenic, chromium, mercury, and some fatty acids can also be treated by precipitation.47 The common precipitating chemicals for metal cations are sulfide, phosphate, hydroxide, or carbonate. Among them, sulfide is the most promising, because sulfides have low solubility over a broad pH range. Precipitation is most applicable to sites with sand or coarse silt strata. 

Redox reactions may cause mobile toxic ions to become either immobile or less toxic. Hexavalent chromium is mobile and highly toxic. It can be reduced to be rendered less toxic in the form of trivalent chromium sulfide by the addition of ferrous sulfate. Similarly, pentavalent (V) or trivalent (III) arsenic, arsenate or arsenite are more toxic and soluble forms. Arsenite (III) can be oxidized to As(IV). Arsenate (V) can be transformed to highly insoluble FeAs04 by the addition of ferrous sulfate. 

Seidel, H., Gorsch, K, Amstatter, K. and Mattusch, J. (2005) Immobilization of arsenic in a tailings material by ferrous iron treatment. Water Research, 39, 4073-4082. 

The aim of this work was to investigate the arsenic mobilization from the tailings material (200 - 500 pg/g As) into the seepage water (up to 3.5 mg/L As) and the process of seepage water effluent forming an immobilized precipitate (up to 8 % As) in the creek. Different analytical methods for the determination of total concentrations and different sequential extraction methods as well as hyphenated techniques for speciation analysis were applied to follow the way of the arsenic in this environment. 

The mobilization of arsenic from the tailings material seems to be a slow and continuos process attributed to reduction of iron phases. The seepage water of the middle source contains arsenite as well as arsenate in high concentrations and seems to be the only water source in contact with the tailings material. The concentrations of arsenic downstream are still high and the immobilization process by precipitation of iron hydroxide and coprecipitation or sorption of arsenic is incomplete. A reason for this may be the slow kinetics of the oxidation process and the transport of fine grained hydroxide particles. These particles are mobile and can bind the arsenic (mainly as arsenate) too. 

The immobilized arsenic in the precipitate is bound only by sorption onto the amorphous iron hydroxides. A sustainable immobilization would need additional action. 

Arsenic occurs primarily in sulphide minerals associated with copper ores, and to a lesser extent with zinc, lead and gold ores. Arsenic is produced as a by-product of the smelting of these metals. Primary arsenic production has now ceased in the USA and Europe, and most arsenic is now imported from China and Mexico. The volatility of arsenic represents a significant concern, and there is at present no known natural mechanism by which arsenic is immobilized in the environment. Anthropogenic activities account for an input of some 19000 tonnes into the atmosphere, compared with 12000 tonnes from natural processes, such as volcanism and forest fires (Ayres and Ayres, 1996). 

Immobilizing DENs within a sol-gel matrix is another potential method for preparing new supported catalysts. PAMAM and PPI dendrimers can be added to sol-gel preparations of silicas " and zinc arsenates to template mesopores. In one early report, the dendrimer bound Cu + ions were added to sol-gel silica and calcined to yield supported copper oxide nanoparticles. Sol-gel chemistry can also be used to prepare titania supported Pd, Au, and Pd-Au nanoparticle catalysts. Aqueous solutions of Pd and Au DENs were added to titanium isopropoxide to coprecipitate the DENs with Ti02. Activation at 500°C resulted in particles approximately 4 nm in diameter. In this preparation, the PAMAM dendrimers served two roles, templating both nanoparticles and the pores of the titania support. 

The process can be used to immobilize heavy metals such as Cd, Zn, Cu, Pb, Ni and Co. Cr(VI) can be reduced by some metal-reducing bacteria to the less toxic and less soluble form Cr(III). Arsenate [As(V)] can be reduced to the more mobile arsenite [As(III)] which precipitates as AS2S3, and is insoluble at low pH. Several laboratory-scale tests (batch and column) are currently available to study the feasibility of this process. However, only a few field tests have been performed to date. Two such tests have been conducted in Belgium, one at a non-ferrous industrial site, where the groundwater was contaminated with Cd, Zn, Ni and Co, and the other which was treated by injection of molasses in order to reduce chromium (VI) to chromium (III). A third demonstration in The Netherlands has been performed at a metal surface treatment site contaminated by Zn. The outcomes of a batch test of a groundwater heavily contaminated by Zn, Cd, Co and Ni are presented in Table 5. The initial sulphate concentration was 506mg/l. With the addition of acetate, a nearly... 

Arsenic S, L Bioreduction/biooxidation following immobilization or dissolution... 

Jing, C., Liu, S. and Meng, X. (2005) Arsenic leachability and speciation in cement immobilized water treatment sludge. Chemosphere, 59(9), 1241-47. 

Absorption is the assimilation of a chemical species into the interior of a solid substance. Absorption may include the migration of solutes into the internal pores of a solid material (Fetter, 1993), 117 or the migration or exchange of atoms within the crystalline structure of a mineral (Krauskopf and Bird, 1995), 150. Some researchers use the generic term sorption to refer to a treatment method where both adsorption and absorption may be involved or if adsorption and absorption cannot be distinguished. Sorption and ion exchange have many important roles in immobilizing arsenic in natural environments (Chapter 3). They... 

Jing, C Korflatis, G.P. and Meng, X. (2003) Immobilization mechanisms of arsenate in iron hydroxide sludge stabilized with cement. Environmental Science and Technology, 37(21), 5050-56. 

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