Environ. Sci. Technol. 2009, 43, 1515–1521
Coprecipitation of Arsenate with Metal Oxides. 3. Nature, Mineralogy, and Reactivity of Iron(III)-Aluminum Precipitates A N T O N I O V I O L A N T E , * ,† MASSIMO PIGNA,† STEFANIA DEL GAUDIO,† VINCENZA COZZOLINO,† AND DIPANJAN BANERJEE‡ Dipartimento di Scienze del Suolo, della Pianta e dell’Ambiente, Universita` di Napoli Federico II, Portici (Napoli), Italy, and Institute of Radiochemistry Forschungszentrum Dresden-Rossendorf, 01314 Dresden, Germany
Received August 8, 2008. Revised manuscript received November 25, 2008. Accepted December 15, 2008.
Coprecipitation involving arsenic with aluminum or iron has been studied because this technique is considered particularly efficient for removal of this toxic element from polluted waters. Coprecipitation of arsenic with mixed iron-aluminum solutions has received scant attention. In this work we studied (i) the mineralogy, surface properties, and chemical composition of mixed iron-aluminum oxides formed at initial Fe/Al molar ratio of 1.0 in the absence or presence of arsenate [As/ Fe+Al molar ratio (R) of 0, 0.01, or 0.1] and at pH 4.0, 7.0, and 10.0 and aged for 30 and 210 days at 50 °C and (ii) the removal of arsenate from the coprecipitates after addition of phosphate. The amounts of short-range ordered precipitates (ferrihydrite, aluminous ferrihydrite and/or poorly crystalline boehmite) were greater than those found in iron and aluminum systems (studied in previous works), due to the capacity of both aluminum and arsenate to retard or inhibit the transformation of the initially formed precipitates into well-crystallized oxides (gibbsite, bayerite, and hematite). As a consequence, the surface areas of the iron-aluminum oxides formed in the absence or presence of arsenate were usually much larger than those of aluminum or iron oxides formed under the same conditions. Arsenate was found to be associated mainly into short-rangeorderedmaterials.Chemicalcompositionofallsamples was affected by pH, initial R, and aging. Phosphate sorption was facilitated by the presence of short-range ordered materials, mainly those richer in aluminum, but was inhibited by arsenate present in the samples. The quantities of arsenate replaced by phosphate, expressed as percentages of its total amount present in the samples, were particularly low, ranging from 10% to 26%. A comparison of the desorption of arsenate by phosphate from aluminum-arsenate and iron-arsenate (studied in previous works) and iron-aluminum-arsenate coprecipitates evidenced that phosphate has a greater capacity to desorb arsenate from aluminum than iron sites. * Corresponding author. Phone: 39 081 253 91 75; fax: 39 081 253 91 86; e-mail:
[email protected]. † Universita` di Napoli Federico II. ‡ Institute of Radiochemistry Forschungszentrum DresdenRossendorf. 10.1021/es802229r CCC: $40.75
Published on Web 01/29/2009
2009 American Chemical Society
Introduction Arsenic is ubiquitous in natural environments (soils, groundwaters, surface waters, sediments), but it has been introduced in elevated concentrations in the biosphere through mining activities and the past usage of arsenic-containing pesticides, herbicides, and fungicides (1-4). Contamination of groundwater by arsenic from natural or anthropogenic sources is of particular concern (1, 4). Many studies have revealed that millions of people already suffer from symptoms such as skin, lung, and liver cancer, skin lesions, and melanosis from consumption of drinking waters with high levels of arsenic (>10 µg L-1) (1, 2). Arsenic is easily sorbed on variable-charge minerals (e.g., metal oxides) and may form precipitates or coprecipitates with aluminum, iron, calcium, and manganese (refs 1-5 and references therein). Arsenate is strongly and efficiently sorbed onto the surfaces of both aluminum and iron oxides, on which it forms inner-sphere complexes, whereas arsenite has a weak affinity for aluminum oxides (1-3, 6 –10). Coprecipitation involving arsenate with aluminum or iron has been extensively studied because this technique appears particularly efficient for the removal of arsenic from polluted waters (1, 3, 11-13). We demonstrated (12, 13) that arsenate coprecipitated with iron or aluminum at different pH values and As/Fe or As/Al molar ratios affected the nature of the final precipitates and was only partially desorbed by phosphate. Robins et al. (11) reported favorable coprecipitation of arsenic from using a mixed Fe(III)-Al solution from polluted waters, rather than iron(III) or aluminum solutions. In natural environments iron and aluminum may coprecipitate, forming mixed iron-aluminum oxides with different chemical, mineralogical, and surface properties [point of zero charge (pzc), size of the particles, surface area] and reactivity as affected by pH, initial Fe/Al molar ratio, aging, temperature, and presence of foreign ligands (14-18). Aluminum coprecipitated with iron retards or inhibits the conversion of the initially formed aluminous ferrihydrite into well-crystallized aluminum and/ or iron oxides (15-17). However, to date neither the effect of arsenate onto the mineralogy and surface properties of mixed iron-aluminum oxides nor their capacity to sorb phosphate and the subsequent possible removal of arsenate from the iron-aluminum-arsenate coprecipitates has received much attention. Phosphate is the anion that may compete very strongly with arsenate for common sites on metal oxides and is then able to replace, at least in part, arsenate previously sorbed (9, 12, 13). Phosphate is usually present in polluted waters. Only recently, Masue et al. (18) studied the sorption and desorption of arsenic on/from mixed aluminum and iron oxides. In this paper we describe (i) the influence of pH and aging period on the mineralogy, surface properties, and chemical composition of coprecipitates formed at initial Fe/Al molar ratio of 1 in the absence or presence of arsenate (initial As/ Fe+Al molar ratio of 0, 0.01, or 0.1) and (ii) the reactivity of precipitates toward phosphate and subsequent desorption of arsenate. The objective of our work was to determine the suitability of mixed Al-Fe precipitates to sequester arsenic during precipitation and to compare the reactivity of different coprecipitates (aluminum-arsenate, iron-arsenate, and iron-aluminum-arsenate) formed under similar conditions.
Materials and Methods
Preparation of Samples. Fresh stock solutions of 0.1 mol L-1 Fe(NO3)3 and 0.1 mol L-1 Al(NO3)3 were mixed in order to VOL. 43, NO. 5, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Synthesis Conditions, Mineralogy, and Surface Area of Precipitatesa synthesis conditions sample b
4Fe+AlR0 4Fe+AlR0.01 4Fe+AlR0.1 7Fe+AlR0 7Fe+AlR0.01 7Fe+AlR0.1 10Fe+AlR0 10Fe+AlR0.01 10Fe+AlR0.1
initial As/Fe+Al molar ratio (R) 0 0.01 0.1 0 0.01 0.1 0 0.01 0.1
initial pH 4.0 4.0 4.0 7.0 7.0 7.0 10.0 10.0 10.0
surface area (m2 g-1)
mineralogy aged for 30 days
aged for 210 days
aged for 30 days
aged for 210 days
F F F G+F F+G F + [Boh] B + (H) + (F) + [G] B + (H) + (F) + [G] F + [Boh]
F [G] F F G + (F) G + (F) F + (Boh) B + H + (G) + [F] B + (H) + (G) + [F] F + (Boh)
174 ( 7 192 ( 8 198 ( 6 164 ( 5 164 ( 7 203 ( 9 68 ( 3 99 ( 5 189 ( 7
126 ( 5 135 ( 6 138 ( 5 80 ( 4 77 ( 3 125 ( 6 10 ( 1 44 ( 2 127 ( 4
c
a Precipitates were formed in the absence or presence of arsenate after 30 or 210 days of aging at 50 °C. b The first number in the symbol for each sample indicates the pH at which the oxide was prepared; Fe+Al indicates the nature of the oxide (mixed iron-aluminum oxide, initial Fe/Al molar ratio of 1); and R0, R0.01, or R0.1 indicates the initial As/Fe+Al molar ratio of each precipitate. c F, ferrihydrite; H, hematite; G, gibbsite; B, bayerite; Boh, poorly crystalline boehmite; parentheses, not predominant; brackets, small amount.
have an initial Fe/Al molar ratio of 1. These solutions were titrated at 20 °C to pH 4.0, 7.0, or 10.0 by adding 0.5 mol L-1 KOH at a feed rate of 2 mL min-1 under constant stirring, in both the absence and presence of arsenate (K2HAsO4) at an initial As/Fe+Al molar ratio (R) of 0, 0.01, or 0.1. The suspensions were adjusted to 1 L, kept in polypropylene containers, and aged for 1 day at 20 °C and then for 30 or 210 days at 50 °C. After every 7-15 days, the pH of the suspensions was adjusted to the initial value. The synthesis conditions and symbols of prepared samples are summarized in Table 1 (see footnoteb). One sample (7Fe+AlAR0.1) was obtained by adding arsenate (R ) 0.1) 30 min after precipitation of iron and aluminum at pH 7.0 and aged for 1 day at 20 °C or for 30 days at 50 °C. After aging, the suspensions were centrifuged at 10000g for 30 min, filtered through Nalgene acetate membrane (pore size
