Arsenic adsorption effects

For a reconstmcted surface, the effect of an adsorbate can be to provide a more bulk-like enviromnent for the outemiost layer of substrate atoms, thereby lifting the reconstmction. An example of this is As adsorbed onto Si(l 11)-(7 X 7) [37]. Arsenic atoms have one less valence electron than Si. Thus, if an As atom were to replace each outemiost Si atom in the bulk-temiinated stmcture, a smooth surface with no impaired electrons would be produced, with a second layer consisting of Si atoms in their bulk positions. Arsenic adsorption has, in fact, been found to remove the reconstmction and fomi a Si(l 11)-(1 x l)-As stmcture. This surface has a particularly high stability due to the absence of dangling bonds. 

Arsenate is readily adsorbed to Fe, Mn and Al hydrous oxides similarly to phosphorus. Arsenate adsorption is primarily chemisorption onto positively charged oxides. Sorption decreases with increasing pH. Phosphate competes with arsenate sorption, while Cl, N03 and S04 do not significantly suppress arsenate sorption. Hydroxide is the most effective extractant for desorption of As species (arsenate) from oxide (goethite and amorphous Fe oxide) surfaces, while 0.5 M P04 is an extractant for arsenite desorption at low pH (Jackson and Miller, 2000). 

Kampf N, Scheinost AC, Schultze DG (2000) Oxides minerals. In Sumner ME (ed) Handbook of soil science, CRC Press, Boca Raton (FL), F125-F168 Jain A, Loeppert RH (2000) Effect of competing anions on the adsorption of arsenate and arsenite by ferrihydrite. J Environ Qual 29 1422-1430 Jain A, Raven KP, Loeppert RH (1999) Arsenite and arsenate adsorption on ferrihydrite surface charge reduction and net OH release stoichiometry. Environ Sci Technol 33 1179-1184... 

Violante A, Krishnamurti GSR, Pigna M (2008) Mobility of trace elements in soil environments. In Violante A, Huang PM and Gadd G (eds) Wiley-JUPAC series on biophysico-chemical processes of metals and metalloids in soil environments. John Wiley Sons, Hoboken, USA Waltham AC, Eick MJ (2002) Kinetic of arsenic adsorption on goethite in the presence of sorbed silicic acid. Soil Sci Soc Am J 66 818-825 Waychunas GA, Fuller CC, Rea BA, Davis J (1996) Wide angle X-ray scattering (WAXS) study of two-line ferrihydrite structure Effect of arsenate sorption and counterion variation and comparison with EXAFS results. Geochim Cos-mochim Acta 60 1765-1781... 

Izumi, F. (1993) Rietveld analysis program RIE-TAN and PREMOS and special applications. In Young, R.A. (ed.) The Rietveld Method, Oxford, Oxford University Press, 236-253 Jackson, B.P. Miller, W.P. (2000) Effectiveness of phosphate and hydroxide for desorption of arsenic and selenium species from iron oxides. Soil Sci. Soc. Am. J. 64 1616-1622 Jain, A. Raven, K.P. Loeppert, R.EI. (1999) Ar-senite and arsenate adsorption on ferrihy-drite Surface charge reductions and net OEI-release stoichiometry. Environ. Sci. Techn. 

Adsorption of As by two humic acids was a function of pH, As speciation, and the humic acid composition (Thanabalasingam and Pickering, 1986). For both humic acids, adsorption of As(V) was slightly greater than As(III). However, the pH effect differed with humic acid composition. Arsenate adsorption by the humic acid with a higher ash and Ca content was a maximum at pH 6, whereas As(III) adsorption was a maximum at pH 8.5. For the lower ash and Ca humic acid, both As(V) and As(III) exhibited broad adsorption maxima between pH 5.5 and 7.5. 

FeClj was then added to the slurries to evaluate the effect of HFO precipitation on As(V) adsorption. Additional synthetic ground water was added to each slurry to make the final volume 500 mL. In one experiment, 10 mg/L Fe(III) was added. The slurries were then equilibrated for 3 days to allow complete precipitation of HFO. NaOH was then added to slowly increase pH. Slurries were equilibrated for 1 day following each addition of 0.1 N NaOH, followed by 0.4 itm filtering of 20 mL aliquots for As(V) and P(V) analysis. Additional FeCl3 was then added to increase the Fe(III) concentration to 100 mg/L and the experiment was repeated as above. Arsenate adsorption for the 100 mg/L Fe(III) concentration was measured for initial As(V) concentrations of 145 and 1000 lig/L. The iron additions of 10 and 100 mg/L are equivalent to 0.1 and 1.0 mg/g of basalt. 

O Reilly, S. E., Strawn, D. G., and Sparks, D. L., 2001, Residence time effects on arsenate adsorption/desorption mechanisms on goethite Soil Science Society of America Journal, v. 65, p. 67-77. 

Swedlund, P. J., and Webster, J. G., 1999a, Adsorption and polymerisation of silicic acid on ferrihydrite, and its effect on arsenic adsorption Water Research, v. 33, p. 3413-3422. 

A particular care was also recommended for the digestion of the lichen material. A systematic comparison of three microwave-decomposition techniques (with quartz vessels) using HNO3 at 170°C demonstrated that a high pressure (85 bar) and an addition of HF was necessary to obtain a complete recovery of As in this material. The use of a high pressure (85 bar) mode and a medium pressure (10 bar) mode without addition of HF led respectively to As contents 35% and 70% lower than the one found with the high pressure/HF method. It was suspected that an adsorption effect of arsenic on the surface of the quartz vessels could have occurred, which was avoided by HF addition. All other sets of data were accepted for certification. 

As mentioned before, the effect of silica on arsenic adsorption by ferric hydroxide has been reported by a number of researchers. Swedlund and Webster... 

DA Clifford and M Wu. Arsenic treatment technology demonstration Predicting the effect of water quality on arsenic adsorption by activated alumina. Report to the Montana State University Water Resources Center, 2001. 

PJ Swedlund, JG Webster. Adsorption and polymerization of silicic acid on ferrihy-drite and its effect on arsenic adsorption. Water Res 33 3414-3422, 1999. Methods for Evaluating Solid Wastes. SW-846, U.S. Environmental Protection Agency, Cincinnati, OH, 1997. 

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). 

On the surface of metal electrodes, one also hnds almost always some kind or other of adsorbed oxygen or phase oxide layer produced by interaction with the surrounding air (air-oxidized electrodes). The adsorption of foreign matter on an electrode surface as a rule leads to a lower catalytic activity. In some cases this effect may be very pronounced. For instance, the adsorption of mercury ions, arsenic compounds, or carbon monoxide on platinum electrodes leads to a strong decrease (and sometimes total suppression) of their catalytic activity toward many reactions. These substances then are spoken of as catalyst poisons. The reasons for retardation of a reaction by such poisons most often reside in an adsorptive displacement of the reaction components from the electrode surface by adsorption of the foreign species. 

Frost R.R., Griffin R.A. Effect of pH on adsorption of arsenic and selenium from landfill leachate by clay minerals. Soil Sci Soc Am J 1977 41 53-57. 

Frost RR, Griffin RA (1977) Effect of pH on adsorption of As and selenium from land fill leachate by clay minerals. Soil Sci Soc Am J 41 53—57 Goh K-H, Lym TT (2005) Arsenic fractionation in a fine soil fraction and influence of various anions on its mobility in the sub surface environment. Appl Geochem 20 229-239... 

Liu F, De Cristofaro A, Violante A (2001) Effect of pH phosphate and oxalate on the adsorption/desorption of arsenate on/from goethite. Soil Sci 166 197-208 Livesey NT, Huang PM (1981) Adsorption of arsenate by soils and its relation to selected properties and anions. Soil Sci 131 88-94 Manceau A (1995) The mechanism of anion adsorption on iron oxides Evidence for the bonding of arsenate tetrahedra on free Fe(0, OH)6 edges. Geochim Cosmochim Acta 59 3647-3653. 

Yoshimura et al. [193] carried out microdeterminations of phosphate by gel-phase colorimetry with molybdenum blue. In this method phosphate reacted with molybdate in acidic conditions to produce 12-phosphomolybdate. The blue species of phosphomolybdate were reduced by ascorbic acid in the presence of antimonyl ions and adsorbed on to Sephadex G-25 gel beads. Attenuation at 836 and 416 nm (adsorption maximum and minimum wavelengths) was measured, and the difference was used to determine trace levels of phosphate. The effect of nitrate, sulfate, silicic acid, arsenate, aluminium, titanium, iron, manganese, copper, and humic acid on the determination were examined. 

Hering, J.G., P.Y. Chen, and J.A. Wilkie. 1997. Arsenic removal from drinking-water by coagulation the role of adsorption and effects of source water composition. Pages 369-381 in C.O. Abernathy, R.L. Calderon, and W.R. Chappell (eds.). Arsenic. Exposure and Health Effects. Chapman Hall, London. 

Effects of Pentavalent Sb Ions on the Adsorption of Divalent Co-57 on Hematite. Benjamin and Bloom reported that arsenate ions enhance the adsorption of cobaltous ions on amorphous iron oxyhydroxide (J 6). Similarly, when divalent Co-57 ions were adsorbed on hematite together with pentavalent Sb ions, an increase of adsorption in the weakly acidic region was observed. For example, when 30 mg of hematite was shaken with 10 cm3 of 0.1 mol/dm3 KC1 solution at pH 5.5 containing carrier-free Co-57 and about 1 mg of pentavalent Sb ions, 95 % of Co-57 and about 30 % of Sb ions were adsorbed. The emission spectra of the divalent Co-57.ions adsorbed under these conditions are shown in Figure 8 together with the results obtained under different conditions. As seen in Figure 8, the spectra of divalent Co-57 co-adsorbed with pentavalent Sb ions are much different from those of Co-57 adsorbed alone (Figure 3). These observations show a marked effect of the.co-adsorbed pentavalent Sb ions on the chemical structure of adsorbed Co-57. 

---