Three iron oxide materials have been studied for uptake of three anions (arsenate, phosphate and fluoride) and a cation (cadmium) from aqueous solutions. Two of the materials were produced using original procedures developed at Loughborough University. The former material was conditioned by a controlled freeze-thaw procedure to enhance granularity and the latter was air-dried at room temperature. Their capacities were compared with a commercially available material supplied by GEH Wasserchemle, Germany. Pore size distributions and specific surface area values were determined by N2 analysis at 77 K. All samples possessed a reasonable specific surface area, in the range 200-300 m2/g and were mesoporous. Samples produced at Loughborough University also contained some macropores, evidence of a more amorphous structure or lack of pH control during production. X-ray diffraction indicated that all samples had some b-FeOOH present and that the chloride content and production pH affected the material crystallinity. Crystallinity increased with increasing chloride content and a higher production pH resulted in the presence of more than one phase. Chemical characterisation was also completed on all three samples. The point of zero net proton charge and isoelectric point for each material was obtained by potentiometric batch titrations and zeta potential measurements respectively. The difference in these values increased with a higher chloride content and all samples studied possessed a positive surface at low pH and negative surface at high pH. These parameters were not greatly affected by the background electrolyte concentration, implying that the background electrolyte is not specifically adsorbed. However, arsenate and phosphate appeared to be specifically adsorbed as the isoelectric point decreased. The uptake capacities for arsenate, phosphate. fluoride and cadmium of all three samples were obtained by measuring batch isotherms at 25 degrees C. The pH range was 4-9, using various initial concentrations up to a maximum of approximately 30 uM. For all anionic species studied, the capacity decreased with increasing pH, and the reverse trend was noted for cadmium. The Langmuir model provided a good fit for the anionic isotherms and the Freundlich model for the cationic isotherms. The materials studied possessed a markedly higher capacity for fluoride than arsenate and phosphate, with an intermediate capacity for cadmium. This indicates that fluoride is attached to the surface via monodentate (single) bonds, whilst both arsenate and phosphate are primarily attached to the surface via bidentate (two) bonds. Cadmium is probably bound by both these mechanisms. The effect of competing anions on arsenic uptake capacity was determined using mini-column experiments of binary (arsenate-fluoride, arsenate-Phosphate and phosphate-fluoride) and ternary (arsenate-fluoride-phosphate) mixtures. Arsenate removal was strongly affected by the presence of phosphate, but was only slightly lower in the presence of fluoride. (Continues...).

The need of making arsenic contaminated ground water potable is urgent in parts of Burkina Faso. An implementation of a treatment design using Granular Ferric Hydroxide (GFH) is under development. Water from a tube-well in Lilgomdé, Yatenga province, Burkina Faso, has been treated with the adsorbent GFH through column experiments. The water had an arsenic concentration varying between 99 and 215 μg/L and an average pH of 7.9. The study has shown that arsenic, predominantly in the form of arsenate, can be adsorbed to the material in significant amounts despite a high natural pH and the presence of ions competing with arsenic for adsorption sites on the GFH. When run through the column, the pH of the effluent water drastically decreased in the beginning. However, the low pH was soon followed by a slower readjustment towards the pH of the influent water. The adsorption of phosphates and fluorides was also studied. Both competitors exist in higher molar quantities than arsenic in the ground water. Even though arsenic displays a higher affinity for the GFH, an average 44 % of total phosphate and 64 % of the fluoride were adsorbed, making them a factor affecting the results of the study. Hydrogen carbonate is also believed to affect the adsorption process but this could not be confirmed. The empty bed contact time (EBCT), describing the average time of contact between the adsorbent and the water, has shown to be of importance. Increasing the EBCT resulted in notably more arsenic being adsorbed per volume GFH. When increasing the contact time, the study showed that reducing the speed of the flow was more effective than increasing the volume of the adsorbent. The GFH was also found to have a self-regenerating ability to a certain extent. When interrupting the experiment and leaving the column material in the aqueous solution for several days, the arsenic adsorption capacity after the break was shown to be higher than just before it. A 13 % increased capacity was shown in one experiment. Conclusively, the results of this study suggest no hindrances towards developing larger scale columns and prototypes to be applied at tube-well pump stations. Further investigations on the treatment method with GFH, on arsenic contaminated water, are recommended.

Phosphonate antiscalants are commonly used in nanofiltration and reverse osmosis water treatment to prevent membrane fouling by mineral scale. In many circumstances it is desirable to remove these phosphonate compounds before concentrate disposal or further treatment. This research investigated the removal of phosphonate compounds from simulated membrane concentrate solutions using ferric hydroxide adsorbents. Two phosphonate antiscalants were investigated, Permatreat 191® (PT191) and nitrilotrimethylphosphonic acid (NTMP). Batch adsorption isotherms and column breakthrough and regeneration experiments were performed on two commercial adsorbents and a ferric hydroxide loaded polyacrylonitrile fiber adsorbent prepared in our laboratory. The best performing adsorbent was Granular Ferric Hydroxide® (GFH) obtained from GEH Wasserchemie. Adsorption isotherms measured after 24-hour equilibration periods showed initial concentration effects, whereby the isotherms were dependent on the initial adsorbate concentration in solution. Significant differences in adsorption behavior were observed between the PT191 and the NTMP adsorbates. Differences in adsorption behavior between NTMP and PT191 are all consistent with the PT191 containing fewer phosphonate functional groups per molecule than NTMP. Desorption rates were bimodal, with 40-50% of the adsorbed phosphonate being released on a time scale of 10-24 hours, while the remaining fraction was released approximately one order of magnitude more slowly. The slow desorbing fraction primarily resulted from equilibrium effects resulting from significant phosphonate adsorption, even in 1.0 mol/L NaOH solutions. Complete regeneration could not be achieved, even after eluting the adsorbent columns with more than 300 bed volumes of 1.0 mol/L NaOH. However, the incomplete regeneration had only a minor effect on phosphonate uptake in subsequent column breakthrough experiments.