Evaluation of natural goethite on the removal of arsenate and selenite from water.pdf (original) (raw)
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Evaluation of natural goethite on the removal of arsenate and selenite from water
Elevated arsenic and selenium concentrations in water cause health problems to both humans and wildlife. Natural and anthropogenic activities have caused contamination of these elements in waters worldwide, making the development of efficient cost-effective methods in their removal essential. In this work, removal of arsenate and selenite from water by adsorption onto a natural goethite (α-FeOOH) sample was studied at varying conditions. The data was then compared with other arsenate, selenite/goethite adsorption systems as much of literature shows discrepancies due to varying adsorption conditions. Characterization of the goethite was completed using inductively coupled plasma mass spectrometry, X-ray diffraction, Fourier-transform infrared spectroscopy, scanning electron microscopy, and Brunauer-Emmett-Teller surface area analysis. Pseudo-first order (PFO) and pseudo-second order (PSO) kinetic models were applied; including comparisons of different regression methods. Various adsorption isotherm models were applied to determine the best fitting model and to compare adsorption capacitates with other works. Desorption/leaching of arsenate and selenite was studied though the addition of phosphate and hydroxyl ions. Langmuir isotherm modeling resulted in maximum adsorption capacities of 6.204 and 7.740 mg/g for arsenate and selenite adsorption, respectively. The PSO model applied with a non-linear regression resulted in the best kinetic fits for both adsorption and desorption of arsenate and selenite. Adsorption decreased with increasing pH. Phosphate induced desorption resulted in the highest percentage of arsenate and selenite desorbed, while hydroxide induced resulted in the fastest desorption kinetics.
Adsorption kinetics of phosphate and arsenate on goethite. A comparative study
Journal of Colloid and Interface Science, 2007
The adsorption kinetics of phosphate and arsenate on goethite is studied and compared. Batch adsorption experiments were performed at different adsorbate concentrations, pH, temperatures and stirring rates. For both oxoanions the adsorption rate increases by increasing adsorbate concentration, decreasing pH and increasing temperature. It does not change by changing stirring rate. The adsorption takes place in two processes: a fast one that takes place in less than 5 min and a slow one that takes place in several hours or more. The rate of the slow process does not depend directly on the concentration of phosphate or arsenate in solution, but depends linearly on the amount of phosphate or arsenate that was adsorbed during the fast process. Apparent activation energies and absence of stirring rate effects suggest that the slow process is controlled by diffusion into pores, although the evidence is not conclusive. The similarities in the adsorption kinetics of phosphate and arsenate are quantitatively shown by using a three-parameters equation that takes into account both the fast and the slow processes. These similarities are in line with the similar reactivity that phosphate and arsenate have in general and may be important for theoretical and experimental studies of the fate of these oxoanions in the environment.
The role of silicate in the adsorption/desorption of arsenite on goethite
Chemical Geology, 2008
The importance of anion displacement as a mechanism for arsenic release was examined through the competitive adsorption and desorption of arsenite on goethite in the presence of silicate. Arsenite remained the only arsenic species throughout the adsorption studies. Oxyanion surface concentration versus time plots developed from adsorption experiments were analyzed with a kinetic rate equation to determine the apparent rate coefficients for arsenite and silicate alone and arsenite in the presence of pre-equilibrated silicate. Silicate adsorption on goethite was significantly slower than arsenite. FTIR analysis indicated that silicate polymerization on goethite may be related to the slow adsorption kinetics. Under all conditions pre-equilibrated silicate reduced the rate and total quantity of arsenite adsorbed. The percent increase in the final aqueous arsenite concentration ranged from 0.25 to 30% depending on the concentration of the adsorbed silicate and aqueous arsenite. Increases in the aqueous arsenite were greatest after 30 min before reaching a steady state after 150 min. Desorption experiments where silicate was introduced to previously adsorbed arsenite indicated that silicate was able to irreversibly displace between 0.3 and 1.5% of the adsorbed arsenite resulting in an increase in solution concentrations ranging from 9 to 266 μg L − 1 of arsenite. Experimental results demonstrate the importance of anion displacement as a mechanism for arsenic release. This is readily seen through the ability of silicate, a naturally occurring and ubiquitous oxyanion, to: 1) reduce arsenite adsorption rates, 2) block potential adsorption sites thereby reducing the total quantity of arsenite adsorbed, and 3) displace adsorbed arsenite. These three processes may ultimately result in an increase in the mobility and potential bioavailability of arsenite.
Adsorption of arsenate on synthetic goethite from aqueous solutions
Journal of Hazardous Materials, 2006
Goethite was synthesized from the oxidation of ferrous carbonate precipitated from the double decomposition of ferrous sulfate doped with sodium lauryl sulfate (an anionic surfactant) and sodium carbonate in aqueous medium. The specific surface area and pore volume of goethite were 103 m 2 g −1 and 0.50 cm 3 g −1 . Batch experiments were conducted to study the efficacy of removal of arsenic(V) using this goethite as adsorbent for solutions with 5-25 mg l −1 of arsenic(V). The nature of adsorption was studied by zeta-potential measurements. The adsorption process followed by Langmuir isotherm and diffusion coefficient of arsenate was determined to be 3.84 × 10 11 cm 2 s −1 . The optimum pH of adsorption was found to be 5.0. The kinetics of adsorption was evaluated with 10 mg l −1 and 20 mg l −1 of As(V) solutions and activation energy of adsorption, as calculated from isoconversional method was in the range of 20 kJ mol −1 to 43 kJ mol −1 . This suggests that the adsorption process is by diffusion at the initial phase and later through chemical control. FT-IR characterization of arsenic treated goethite indicated the presence of both As O Fe and As O groups and supported the concept of surface complex formation.
Desorption kinetics of arsenate from kaolinite as influenced by pH
Journal of environmental quality, 2005
Arsenic is highly toxic and therefore represents a potential threat to the environment and human health. The mobility and bioavailability of arsenic in soil is mostly controlled by adsorption and desorption reactions. Even though adsorption and traditional batch desorption experiments provide information about the environmental fate of As, the equilibrium conditions imposed in these studies would usually not be reached in the natural environment. Flow-through desorption techniques, where the desorbed species are removed from the substrate, can therefore be used to provide information about the rate and mechanisms of As desorption. The effect of pH on As adsorption reactions is relatively well understood; however, desorption of As and the effect of pH on As desorption remain unexplored. Desorption of As(V) (the most dominant arsenic species in aerated soils) was therefore investigated using batch and flow-through desorption experiments. Traditional batch desorption experiments undere...
Normalization, comparison, and scaling of adsorption data: Arsenate and goethite
Journal of Colloid and Interface Science, 2009
Various approaches for analyzing adsorption data were examined to determine the best method for reporting and interpreting the results of adsorption experiments and ultimately extrapolating laboratory measurements to the field. The interactions of arsenate and goethite were used as representative adsorbate and adsorbent, respectively, although the general principles are applicable to other adsorbate–adsorbent systems as well. A modeling exercise was conducted first to determine the theoretical principles governing the comparison and scaling of adsorption data. These principles were then tested on a suite of experimental data, both new and previously published. LogKDLogKD is significantly more sensitive to variations in adsorbate (AsT) or adsorbent (FeT) concentrations than either adsorbed concentration (q) or percentage adsorbed. The sensitivity of KDKD relative to q occurs due to the non-linearity of the adsorption isotherm at a given pH, since as the equilibrium aqueous concentration approaches zero, q also approaches zero while KDKD approaches infinity. Varying AsT and FeT while keeping AsT/FeT fixed yields more consistent values of percentage adsorbed, logKDlogKD, and q, although the adsorbate-to-adsorbent ratios used in laboratory studies often have a rather narrow range compared to those possible in the field. Specific surface area is also a better scaling parameter than the mass of adsorbent, especially between systems with differing adsorbents with markedly different specific surface areas (e.g., natural versus synthetic goethite). Our results have significant implications to contaminant transport modeling, as the constant KDKD approach is the most common method of modeling contaminant transport, while contaminant concentrations in the field are typically low, precisely the conditions where KDKD is most sensitive.In this work we examine the factors governing the normalization, comparison, and scaling of adsorption data using arsenate and goethite as a model system. Using a theoretical model and experimental data the article highlights the advantages and limitations of the various approaches commonly adopted for analyzing adsorption data.
Effect of major anions on arsenate desorption from ferrihydrite-bearing natural samples
Applied Geochemistry, 2008
The influence of background electrolytes (Na 2 HPO 4 Á 2H 2 O, NaHCO 3 , Na 2 SO 4 , NaNO 3 and NaCl) on arsenate (As(V)) desorption from 3 environmental samples (a tailings sample, a stream-bed sediment and a top soil) containing ferrihydrite as the main As-bearing phase has been studied by means of kinetic batch experiments and geochemical simulations. The experimental results indicate that As(V) release increases greatly in the presence of dissolved phosphate and carbonate species. Similarly to PO 3þ 4 , a strong surface interaction of inner-sphere type between ferrihydrite and aqueous carbonate species is suggested. Nitrate and Cl À have negligible effects on the As(V) desorption reaction, whereas SO 2À 4 exhibits intermediate behavior depending on its dissolved concentration that probably influences the type of surface complex (i.e. outer-sphere or inner-sphere). The process of As(V) release follows the first-order rate equation of Lagergren modified for desorption; most values of the desorption rate constant k des are in the range of 0.0012-0.0030 min À1 . Modeling of the desorption experiments with PHREEQC, with ferrihydrite as the main As-bearing phase, indicates that the influence of pH is notably less important than the displacement action of carbonate species in determining the amount of As(V) released to solution. Simulation of As(V) desorption totally fails when the carbonate surface complexes are excluded from the model. In the NaHCO 3 experiments with the tailings sample the best match between observed and calculated data is obtained also including dissolution of scorodite and arsenopyrite in the model. Moreover, modeling has stressed the poor quality of the adsorption constants for sulfate species that leads to strong overestimation of As(V) desorption at pH 4 and underestimation at pH 7.5. Although the findings of this study are consistent with the results of recent studies from other authors, they cannot be generalized or directly applied to natural systems. However, environmental implications concerning As mobility, as well as possible application in various fields (e.g. irrigation agriculture, soil decontamination, water treatment and mine site remediation), might be derived from these findings.
Chemical Geology, 2014
Metal hydroxides (e.g. ferrihydrite) present in geomedia play significant roles in regulating the environmental mobilities of arsenate (As(V)) and inorganic phosphate (P i ) because of their high adsorption affinities for these oxyanions. In this study, results are presented of experiments aimed at determining individual and competitive adsorption/desorption kinetics of As(V) and P i on ferrihydrite at pH 4 and 8. Selected samples were also subjected to As K-edge EXAFS study for understanding the changes with time in As(V) complexation on ferrihydrite in the presence/absence of P i . Both oxyanions showed similar behavior in single ion adsorption experiments. However, when both oxyanions were loaded together, more As(V) was adsorbed than P i . Furthermore, more preequilibrated P i was desorbed by sequentially added As(V) than vice versa. Interactions of As(V) and P i with ferrihydrite slowed down after the initial rapid adsorption/desorption. The experimentally determined adsorption/ desorption kinetic data for As(V) and P i showed good compliance with pseudo-second order, Elovich, and powerfunction equations. Both oxyanions competed for adsorption on ferrihydrite, and each of them showed a limited capacity to desorb the other. EXAFS analysis of selected samples indicated the presence of mononuclear (2E) and binuclear (2C) bidentate As(V) surface complexes. The Fe coordination numbers (CN) increased with increasing time and decreased with addition of P i into the system. A higher proportion of Fe CN associated with 2E As(V) surface complexes decreased after the addition of P i , compared to Fe CN associated with 2C As(V) surface complexes. The competitive desorption study indicates that the excessive input of P i due to the overuse of fertilizers could mobilize As(V) from contaminated geomedia. Furthermore, insights into P i -induced desorption of As(V) could also provide an opportunity for developing chemical treatment methods to intercept the mobilized As(V) by co-precipitation in apatite-like phases.
Zastita materijala, 2014
Adsorption of arsenites and arsenates from aqueous solutions by application of modified natural inorganic materials-a kinetic study Two commercially available adsorbents, white tuff from Strmos region and a material with a commercial name Zeofit obtained from Palanka region, were investigated as possible raw materials for adsorption of arsenites and arsenates from aqueous solutions. These raw materials were modified through chemical insertion of iron within the basic material structure and ionic substitution of calcium and magnesium ions with sodium ions in order to improve the sorptive and ion-exchange properties of studied raw materials. Obtained results from the experimental study of the adsorption of arsenic ions from simulated aqueous solutions suggest that the Zeofit material produced better results than the white tuff material, which showed satisfactory adsorption efficiency only at lower arsenic ions concentrations. Drinking water from the water supply system in Skopje was used for preparation of the studied arsenic ions solutions. Performed kinetic study and resulting kinetic models for both materials, generated a kinetic model of first order for the white tuff material, whereas the Zeofit material adsorption kinetics indicated a pseudosecond order.