Laura Borgnino

The role of Fe(III) modified montmorillonite on fluoride mobility: Adsorption experiments and competition with phosphate

Fluoride adsorption onto Fe(III) modified montmorillonite was investigated using batch experiments. The effect of reaction time, pH, ionic strength and phosphate, as a competitive anion, was evaluated. Kinetics indicated that adsorption obeys a pseudo-first-order rate law which involves two steps. The fast one (bulk transport/surface reaction) occurs instantaneously. The slower (diffusion in pores) takes hours to complete. The adsorption rate increases by increasing the fluoride concentration and by decreasing pH. The presence of phosphate reduces fluoride adsorption and reveals that both ions are in competition for surface sites. The reduction in fluoride adsorption when phosphate is present depends on the order of adsorbate addition. The higher fluoride adsorption occurs when both anions are added simultaneously, whereas when either fluoride or phosphate is added first, the fluoride adsorption is lower. The presence of fluoride does not have a measurable effect on phosphate adsorption. The results obtained contribute to our understanding of the mobility of fluoride in surface water which has naturally high levels of fluoride, in both the presence and absence of phosphate.

Arsenate adsorption at the sediment–water interface: sorption experiments and modelling

Arsenate adsorption was studied in three clastic sediments, as a function of solution pH (4.0–9.0) and arsenate concentration. Using known mineral values, protolytic constants obtained from the literature and Kads values (obtained by fitting experimental adsorption data with empirical adsorption model), the constant capacitance surface complexation model was used to explain the adsorption behavior. The experimental and modelling approaches indicate that arsenate adsorption increases with increased pH, exhibiting a maximum adsorption value before decreasing at higher pH. Per unit mass, sample S3 (smectite–quartz/muscovite–illite sample) adsorbs more arsenate in the pH range 5–8.5, with 98% of sites occupied at pH 6. S1 and S2 have less adsorption capacity with maxima adsorption in the pH ranges of 6–8.5 and 4–6, respectively. The calculation of saturation indices by PHREEQC at different pH reveals that the solution was undersaturated with respect to aluminum arsenate (AlAsO42H2O), scorodite (FeAsO42H2O), brucite and silica, and supersaturated with respect to gibbsite, kaolinite, illite and montmorillonite (for S3 sample). Increased arsenate concentration (in isotherm experiments) may not produce new solid phases, such as AlAsO42H2O and/or FeAsO42H2O.

CHAPTER 1:Fluoride in the Context of the Environment

An estimated 70 million people around the world suffer from fluorosis due to chronic exposure to high levels of fluoride in drinking water. The sources of this element in water are mostly geogenic, although important contributions also come from industrial activities and coal burning. Fluorine is the 13th most abundant element in the Earths crust, as it is contained in several rock-forming minerals. Among these, micas, apatites and fluorite are the most common minerals responsible for the release of elevated concentrations of fluoride in natural waters. Fluoride is also commonly associated with volcanic activity, which on a global scale may release important amounts of gaseous fluoride compounds to the atmosphere and produce large deposits of F-rich lavas and volcanic ashes. The mobility of fluorine in aqueous reservoirs depends on the interplay of a number of geochemical processes that determine its removal or release into the solution. The main processes that affect the dynamics of fluoride in natural environments are the dissolution and precipitation of F-bearing minerals and the adsorption/desorption from metal (hydr)oxides and clay minerals. Most of the worlds high-fluoride districts coincide with areas affected by volcanic activity, regions underlain by crystalline igneous and metamorphic rocks, and large sedimentary basins in arid and semiarid conditions. Critical zones include the Pacific volcanic belt, cratonic areas in central Africa, Asia and North and South America, the East African Rift valley, the large sedimentary basins in southern South America, China and the arid region on the border between USA and Mexico.

Phosphate concentration and association as revealed by sequential extraction and microprobe analysis: The case of sediments from two Argentinean reservoirs

[1] This article reports the general characteristics of the sediments of two Argentinean reservoirs, which are used for water supply. The chemical composition, granulometry, and specific surface area are presented together with a study of total phosphate concentration and phosphate association by combining sequential extraction and microprobe analysis. In general, the sediments of both reservoirs have similar characteristics. Sequential extraction reveals that the main P fractions in the studied sediments are Ca-bound phosphate in river mouths and Fe-bound phosphate in the rest of the reservoir stations. Microprobe analysis appears to be an important complementary technique to sequential extraction. Combined with chemical fractionation and specific surface area measurements, these analyses indicate that Ca-bound phosphate is mainly distributed within grains or particles highly concentrated in Ca and P, whereas Fe-bound phosphate is rather homogeneously distributed in the sediments at the surface of fine particles. Microprobe analyses also revealed an important coating of phyllosilicate surfaces with Fe (hydr)oxides, which explains the good correlation found between Fe-bound phosphate, clay fraction, and specific surface area. The role of sediments as a potential source of phosphate is discussed in terms of P association in the solid phase and dissolved oxygen concentration profiles in the water column.

Mechanisms of arsenic and fluoride release from Chacopampean sediments (Argentina)

We have examined the sources and mechanisms that control the release of fluoride and arsenate from fluvial and loess sediments collected from the Chacopampean region, Argentina. The two types of sediment show slight differences regarding the mechanism of release. In fluvial sediments the release of fluoride seems to be controlled by the dissolution of primary F-bearing minerals, such as fluorapatite (FAp) and biotite. Much lower concentrations of As are also released from fluvial sediments under acidic conditions, owing to the dissolution of As-bearing igneous FAp. Under more alkaline conditions, the release of both anions is the result of desorption from Fe (hydr)oxide coats. In loess sediments, F and As are released by dissolution of FAp present either as detritic grains or forming thin coats onto the volcanic ash, while arsenate is preferentially desorbed from Fe (hydr)oxide coats under alkaline conditions or by competitive desorption at acidic pH.

Continuous flow method for the simultaneous determination of phosphate/arsenate based on their different kinetic characteristics.

This paper proposes a new automated spectrophotometric method for the simultaneous determination of phosphate and arsenate without pre-treatment, which is faster, simpler, less expensive and hazardous than other well-known methods used with water samples. Such method is based on the different kinetic characteristics of complex formation of phosphate and arsenate with ammonium molybdate. A flow system was used in order to achieve good mixing and to provide precise time control. All the measurements were performed at the isosbestic point wavelength (885 nm). Chemical variables were optimized by factorial design (ammonium molybdate 0.015 mol L(-1), potassium antimony tartrate 1×10(-4) mol L(-1), and sulphuric acid 0.7 mol L(-1)). An appropriate linear range for both analytes (0.50-8.00μmolL(-1)), good inter-day reproducibility (4.9% [P] and 3.3% [P+As]) and a sample throughput of 6h(-1) were obtained. The detection limits are 0.4 μmol L(-1) P and 0.19 μmol (-1) [P+As] (3.3S y/x). The method was validated.

Long term metal release and acid generation in abandoned mine wastes containing metal-sulphides

The sulphide-rich mine wastes accumulated in tailing dumps of La Concordia Mine (Puna of Argentina) have been exposed to the weathering action for more than 30 years. Since then, a series of redox reactions have triggered the generation of a highly acidic drainage -rich in dissolved metals-that drains into the La Concordia creek. The extent of metal and acid release in the site was analysed through field surveys and laboratory experiments. Static tests were conducted in order to predict the potential of the sulphidic wastes to produce acid, while Cu-, Zn-, Fe- and Pb-bearing phases present in the wastes were identified by XRD, SEM/EDS analysis and sequential extraction procedures. Finally, the release of these metals during sediment-water interaction was assessed in batch experiments carried out in a period of nearly two years. Field surveys indicate that the prolonged alteration of the mine wastes led to elevated electrical conductivity, pH values lower than 4 and metal concentrations that exceed the guide values for drinking water in the La Concordia stream regardless of the dominating hydrological conditions. The highly soluble Fe and Mg (hydrous)sulphates that form salt crusts on the tailings surfaces and the riverbed sediments play an important role in the control of metal mobility, as they rapidly dissolve in contact with water releasing Fe, but also Cu and Zn which are scavenged by such minerals. Another important proportion of the analysed metals is adsorbed onto Fe (hydr)oxides or form less soluble hydroxysulfates. Metals present in these phases are released to water more slowly, thus representing a potential long term source of heavy metal pollution. The obtained results are a contribution to the understanding of long term metal transformations and mobility in mine waste-impacted sites.