Sorption of phosphates and arsenic

Sorption of Phosphate and Arsenic Species by Anaerobic Sediments... 

This sorption material was suggested by Peraniemi et al. [2] for analytical enrichments of phosphate and arsenate. The activated carbon was shaken with a solution of Zr0(N03)2 as described in [2] and a final concentration of 28 mg Zr per g activated carbon was yield. 

The sorption of phosphate and the arsenic species on the twelve soils used in Wauchope s experiment correlated well with both the clay content and the iron content of the soils. The Menominee River sediments, however, were anaerobic, so iron should have been present as Fe(0H)2 rather than the Fe(0H)3 which was, presumably, present in Wauchope s alluvial soils. 

Fig. 10. Sorption of phosphate (P04) (A) and desorption of arsenate (As04) (B) from two samples formed at pH 7.0 and R = 0.1, obtained coprecipitating aluminum and arsenate (7R0.1) or by adding arsenate (7AR0.1) immediately after the precipitation of aluminum. Reaction time was 24 hours. Redrawn from Violante et al. (2006).

Figure 5.8. (a) Effect of contact time on the sorption of phosphate (PO4) and arsenate (ASO4) on an Andisol at pH 5.0. The oxyanions were added alone (filled symbols) or as a mixture at an initial arsenate/phosphate molar ratio of 1 (open symbols). (6) Effect of contact time on rf (rf = sorbed AsO4/sorbed PO4 molar ratio). (Modified from Violante and Pigna, 2002.)... 

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). 

Figures 3 and 4 show the amounts of arsenate sorbed onto ferrihydrite and Al(OH)x after 24 hrs of reaction with a surface coverage of arsenate and phosphate respectively of 50 or 100%. The numbers in parenthesis indicate the efficiency (in percentage) of phosphate in preventing sorption of arsenate calculated according to the expression of Deb and Datta (1967). 

Fig. 3. Sorption of arsenate (ASO4) onto ferrihydrite or Al(OH)x in the presence of phosphate (PO4) or phosphate and malate (Mai) at 50% surface coverage of arsenate and at initial ASO4/PO4 molar ratio of 1.0 or 0.5. Arsenate and phosphate were added as a mixture (As04+P04 As04+2P04) or phosphate was added 24 hrs before arsenate (P04 before As04) or arsenate was added 24 hrs before phosphate (As04 before P04). Arsenate, phosphate and malate were added as a mixture (As04+ P04/Mal molar ratio of 1). The numbers in parenthesis indicate the effectiveness of phosphate in preventing arsenate sorption. From Del Gaudio (2005).

Fig. 6. Kinetics of arsenate (As04) sorption onto ferrihydrite (A) or Al(OH)x (B) at pH 5.0 in the absence or presence of phosphate (P04) or phosphate and malate (Mai). Initial PO4/ASO4 molar ratio of 1 (As04 + P04) and AsO4 + PO/Mal molar ratio of 1 (As04+P04 +2Mal). Arsenate was added at 50% of surface coverage (authors unpublished data, 2007).

O Reilly et al. (2001) studied the effect of sorption residence time on arsenate desorption by phosphate (phosphate/arsenate molar ratio of 3) from goethite at different pH values. Initially, desorption was very fast (35% arsenate desorbed at pH 6.0 within 24 hrs) and then slowed down. Total desorption increased with time reaching about 65% total desorption after 5 months. These authors found no measurable effect of aging on desorption of arsenate in the presence of phosphate. Furthermore, desorption results at pH 4.0 were similar to the desorption behaviour at pH 6.0. On the contrary, Arai and Sparks (2002) demonstrated that the longer the residence time (3 days-1 year), the greater was the decrease in arsenate desorption by phosphate from a bayerite. 

Pigna M, Colombo C, Violante A (2003) Competitive sorption of arsenate and phosphate on synthetic hematites (in Italian). Proceedings XXI Congress of Societa Italiana Chimica Agraria SICA (Ancona), pp 70-76 Quirk JP (1955) Significance of surface area calculated from water vapour sorption isotherms by use of the B. E. T. equation. Soil Sci 80 423-430 Rancourt DG, Fortin D, Pichler T, Lamarche G (2001) Mineralogical characterization of a natural As-rich hydrous ferric oxide coprecipitate formed by mining hydrothermal fluids and seawater. Am Mineral 86 834-851 Raven K, Jain A, Loeppert, RH (1998) Arsenite and arsenate adsorption on ferrihydrite kinetics, equilibrium, and adsorption envelopes. Environ Sci Technol 32 344-349... 

As further discussed in Chapters 2 and 7, the sorption of arsenic on iron (oxy)(hydr)oxides is very sensitive to pH and competing anions, such as phosphate and sulfate (Goh and Lim, 2004). In general, the sorption of inorganic As(V) decreases as pH values rise from 3 to 10 (Su and Puls, 2001, 1489). H2As04 is the dominant As(V) ion at pH 3-6. At pH <6, the surfaces of iron (oxy)(hydr)oxides usually have net positive charges (i.e. they are below their zero points of charge (ZPCs) and isoelectric points ... 

As discussed earlier in Section 3.17, the excessive application of arsenic-bearing pesticides and phosphate fertilizers on agricultural lands, golf courses, and lawns may locally contaminate surface waters and ground-waters (Welch et al., 2000), (Lewis et al., 2002), 590. Phosphates desorb arsenic from mineral surfaces and readily interfere with the sorption and coprecipitation of arsenic onto iron (oxy)(hydr)oxides (Campos, 2002). Commercial phosphate fertilizers also frequently contain >13 mg kg-1 of arsenic impurities (Campos, 2002), which may further contribute to groundwater contamination. 

Reynolds, J.G., Naylor, D.V. and Fendorf, S.E. (1999) Arsenic sorption in phosphate-amended soils during flooding and subsequent aeration. Soil Science Society of America Journal, 63(5), 1149-156. 

Su and Puls (2001b) investigated possible interferences from different oxyanions on the sorption of As(V) and As(III) on Peerless zerovalent iron. Using chloride as a relatively nonreactive standard, Su and Puls (2001b) found that phosphate had the greatest effect in decreasing arsenic sorption on zerovalent iron. This observation is consistent with other batch (Kanel et al., 2005) and column (Su and Puls, 2003) studies. Like As(V), phosphate can form inner sphere complexes (Chapter 2) or produce distinct precipitate layers on zerovalent iron surfaces (Su and Puls, 2001b, 4564). 

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