Arsenic removal from water using seawater-neutralised red mud (Bauxsol)

Hülya Genc Fuhrman

Research output: Book/ReportPh.D. thesis

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Abstract

This thesis presents the results of a detailed study on “arsenic removal from water using seawater-neutralised red mud (Bauxsol™)”, and incorporates six papers that describe and discuss the experimental work carried out as part of the PhD project.

Increasingly stringent legislation on the permissible concentrations of arsenic in
drinking water has led to increased investigations of the occurrence, chemical speciation and mobility of arsenic in natural waters and of methods for removing arsenic during water treatment. Epidemiological studies suggest that there are significant health risks, including cancer, associated with prolonged exposure to elevated arsenic concentrations in drinking water even at quite low concentrations. Although background arsenic concentrations in natural environments are usually low, arsenic concentrations are high in many parts of the world due to mobilisation from natural geological sources or at a smaller scale from industrial pollution. While arsenic associated with industrial pollution can be managed by improving process engineering and environmental management practices, making water that has a naturally high arsenic content safe to drink requires some form of water treatment to reduce arsenic concentrations. This study addresses the water treatment approach and focuses on developing new arsenic removal technologies that use the red mud residues from bauxite refineries as a new sorbent.

Arsenic can exist in natural waters in both organic and inorganic forms, however only the inorganic forms, arsenate (As(V)) and arsenite (As(III)), are considered in this study because all available data indicate that the amount of organic arsenic in drinking water sources is insignificant. The experiments carried out during this study address the removal of both arsenate and arsenite using Bauxsol and new sorbents developed during the study as derivatives of Bauxsol. The study examines both arsenate and arsenite, but the greatest emphasis is placed on the arsenate because both the published literature and the preliminary part of this study indicate that it is very difficult to adsorb uncharged arsenite at near neutral pH values and that the simplest approach is to oxidise arsenite to arsenate for efficient sorption.

In this study seawater-neutralised red mud (Bauxsol) is used under a wide range of experimental conditions and it is found that Bauxsol removes arsenate much more effectively than unneutralised red mud. Neutralisation of the caustic red mud is an essential step in developing an effective arsenic adsorbent from red mud. Seawaterneutralisation increases the calcium content of red mud and data obtained during this study show that the presence of calcium has a positive effect on the arsenic removal. Seawater-neutralisation also decreases the pH of red mud from about 13 to 8.2 - 8.8. The removal of arsenate from water using Bauxsol is sensitive to several parameters tested, including pH, ionic strength, adsorbent dosage, initial arsenate concentration and the source water composition. Arsenate is an anion in a wide pH range, and its increasing adsorption with decreasing pH indicates ligand-like adsorption. The adsorption is independent of ionic strength, suggesting the formation of inner-sphere complexes. Lower arsenate concentrations and higher adsorbent dosages enhanced arsenate removal, but high adsorbent dose rates are not practical. Because the experiments are carried out using deionised water, the effect of other ions likely to be present in potable water is also investigated. Tests conducted with added Ca2+, HCO3-, or Clshowed that under the experimental conditions used, Ca2+ increases arsenate removal possibly due to the increase in the positive charges on the Bauxsol surface, while HCO3- reduces arsenate removal, and Cl- has a negligible effect on arsenate removal. This part of the study concluded that Bauxsol had a good arsenate removal capacity when sufficient adsorbent was used i.e. > 5 g/L, but it is not yet able to compete with other widely used adsorbents. Thus, either Bauxsol could be used as a cost-effective pre-treatment method before applying other more costly arsenate removal methods, or its sorptive capacity could be increased to make it competitive with other conventional sorbents.

The possibility of increasing the arsenate sorption capacity of Bauxsol is investigated in the second part of this study, where acid treatment, combined acid and heat treatment and addition of ferric or aluminum sulfate are tested. When acid treatment or combined acid and heat treatment are applied, the arsenate removal capacity of Bauxsol is significantly increased together with the reactive surface area of the sorbent. Of these acid and heat treated Bauxsol, herein named activated Bauxsol (AB), was the most effective possibly because the heat treatment allowed the Bauxsol to develop more porosity. Unexpectedly, the addition of ferric or aluminum sulfate reduced the arsenate removal capacity of the sorbent. Several reasons are proposed for the reduction of arsenate removal when ferric or aluminum sulfate are added, but it is most likely caused
by the formation of a gelatinous precipitate that occludes some of the potential sorptive sites; the use of ferric chloride may provide an alternative worthy of future investigation. In this study the results clearly indicate that, of the sorbents tested, AB has the highest affinity for arsenate and that AB can perform very effectively even in the presence of competing anions including, phosphate, silicate, sulfate, and bicarbonate. With the promising results obtained for using AB in this part of the study, further investigations were designed to elucidate the sorptive characteristics of the AB adsorbent to understand how it worked and to optimise its performance.

In the third stage of this study, detailed laboratory investigations were carried out to develop an understanding of arsenic removal using AB; in this work the removal of arsenite by AB is studied in addition to the removal of arsenate. Arsenic removal is tested under different pH, adsorbent dosage, initial arsenic concentration, temperature, ionic strength and particle size conditions. As with ordinary Bauxsol, arsenate removal using AB is favoured by decreasing pH, and ionic strength had minor effect, suggesting ligand-like adsorption and inner-sphere complex formation, respectively. Higher temperatures favoured arsenate removal, whereas initial arsenate concentration had no effect on the removal efficiency, and the adsorbent particle size had only a minor effect. When the adsorbent dosage is increased, arsenate removal also increased, but it was found that if arsenic exists in the arsenate form, adsorbent dosages as low as 0.4 g/L are enough to achieve WHO standards 0.01 mg/L under pH and initial arsenate concentration conditions similar to real life conditions. Arsenite adsorption on the other hand, is favoured by slightly alkaline pH values with maximum adsorption recorded at pH 8.5; arsenite removal decreased with increasing initial arsenite concentration. Overall, AB was found to be a very effective adsorbent especially for arsenate removal from water with a sorptive capacity comparable to other conventional sorbents.

The effect of the source water composition on the effectiveness of an absorbent is also important and thus the fourth phase of the study investigates the possible influence of anions in the water on arsenic removal efficiency. The study investigated the influence of phosphate, silicate, sulfate and bicarbonate when present separately and in combination; the tests were conducted at several arsenate and anion concentrations and solution pH values. The results obtained were in agreement with data published elsewhere and indicate the important effect of source water composition on arsenic removal. All tested anions suppressed the arsenic removal with a decreasing order on molar basis of phosphate > silicate > sulfate > bicarbonate. Moreover, when initial arsenate concentration is increased the anion suppression is also increased, and when
the combined effects of the anions are tested it is found that despite the insignificant effect of bicarbonate and sulfate when added alone, they have a larger suppression effect when they coexist with phosphate and silicate.

Because both Bauxsol and AB are produced from an industrial residue, these sorbents could introduce unwanted contaminants to the water. A wide range of elements was investigated, and it is found that neither Bauxsol nor AB caused any secondary pollution of the water as a consequence of the treatment to remove arsenic. Moreover, desorption studies indicated that the bound arsenate can not be easily leached out; a maximum desorption of only 40% could be achieved and that required raising the pH to 11.6 (much less arsenate could be desorbed under lower pH conditions). Despite their high arsenic removal efficiency, neither AB nor Bauxsol would be practical sorbents if they became toxic after use because the disposal of the toxic waste would introduce additional costs and environmental problems. Therefore, the toxicity characteristic leaching procedure (TCLP) test was applied to the spent sorbents, and the results indicated that neither of them was toxic; indeed the amount of arsenic that could be released during the TCLP leach test was exceptionally low. This finding is considered to be particularly important in view of the possible application of the sorbent.

In the final part of the study, column experiments were conducted to test the sorbents under continuous flow conditions and for this part of the study two new sorbents were developed from Bauxsol and AB. The new products, Bauxsol coated sand (BCS) and AB coated sand (ABCS), were developed because the fine texture of Bauxsol and AB makes them difficult to use in column studies. The results of this work show that higher sorptive capacities are evident in column experiments compared to the batch tests. Thus, using BCS or ABCS for water treatment purposes under continuous flow conditions is particularly promising for practical water treatment applications. Higher bed volumes can be achieved before breakthrough. It is partly possible to desorb arsenate from the BCS and ABCS, which is a prerequisite for regeneration, but detailed BCS and ABCS regeneration studies have not yet been carried out.

Among the various technologies available for arsenic removal, adsorption has recently emerged as the most favoured option, as it is easily applicable in small scale and offers endless possibilities of developing cost-effective new adsorbents. In this context, this study has shown that Bauxsol, AB, BCS and ABCS are highly effective new adsorbents especially for As(V) removal. However, full scale field trails using Bauxsol, AB, BCS and ABCS are still required to evaluate their suitability for use under practical conditions.
Original languageEnglish
Place of PublicationKgs. Lyngby
PublisherEnvironment & Resources DTU. Technical University of Denmark
Number of pages51
Publication statusPublished - 2004

Projects

The use of Sewater Neutralised Red-Mud (Bauxsol) to remove Arsenic from Groundwater

Genc-Fuhrman, H., Tjell, J. C., McConchie, D., Schuiling, O., Ledin, A., Ahmed, M. F. & Østergaard, P. H.

Privatist

01/01/200126/05/2004

Project: PhD

Cite this

Genc Fuhrman, H. (2004). Arsenic removal from water using seawater-neutralised red mud (Bauxsol). Environment & Resources DTU. Technical University of Denmark.