On the Conversion of Bauxite Ores to Highly Activated Alumina Media for Water Remediation
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Lawrence Berkeley National Laboratory Recent Work Title On the Conversion of Bauxite Ores to Highly Activated Alumina Media for Water Remediation Permalink https://escholarship.org/uc/item/4tr367p5 Journal Advanced Sustainable Systems, 3(7) ISSN 2366-7486 Authors Haddad, AZ Pilgrim, CD Sawvel, AM et al. Publication Date 2019-07-01 DOI 10.1002/adsu.201900005 Peer reviewed eScholarship.org Powered by the California Digital Library University of California On the Conversion of Bauxite Ores to Highly Activated Alumina Media for Water Remediation Andrew Z. Haddad,* Corey D. Pilgrim, April M. Sawvel, James Nathan Hohman, and Ashok J. Gadgil 1. Introduction Good quality drinking water is necessary to maintain a high quality of life. Skeletal fluorosis is a debilitating condi- Millions lack access to clean and safe drinking water, and current trends tion endemic in 20 countries,[1–4] affecting suggest that billions will face acute water shortages in the coming decades. an estimated 200 million people predomi- Development of new materials has led to technological impacts on water nantly in rural, semi-arid regions of eastern purification, from desalination membranes to atmospheric water scavenging. Africa, southern Asia, and India.[5,6] It is a However, the most challenging aspect of technological solutions is cost: result of drinking water with high fluo- if the community being serviced cannot afford the solution, it is not likely ride concentrations (>10 ppm) and most severely impacts children. Most fluoride to be sustainable. Repurposing Earth-abundant materials to replace highly contamination is naturally occurring, origi- engineered solutions is an atractive solution. Herein, minimal processing nating in groundwater from the minerals of bauxite rocks produces a high-porosity and reactive activated alumina in sellaite (MgF2), fluorspar (CaF2), cryolite situ, without purification directly from the ore. This acid-treated, thermally (Na3AlF6), and fluorapatite (Ca5(PO4)3F), [7] activated bauxite (ATAB) exhibits a high surface area of 401 ± 6 m2 g−1, a among others. Remediation is commonly achieved via reverse osmosis, or related 40-fold increase relative to its parent ore, and a 2× increase relative to the technological solutions in developed state-of-the-art fluoride adsorbent, activated alumina. The composition, countries, but energy-intensive indus- preparation, and mechanism of adsorption are studied by X-ray diffraction, trial methods are often not sustainable X-ray photoelectron spectroscopy, and multiple-quantum magic-angle in remote, resource-poor, or rural areas spinning 27Al nuclear magnetic resonance (NMR). The maximum adsorption for reasons varying from the skilled labor density of ATAB is comparable with that of activated alumina, but ATAB requirements, lack of scalability, inherently high cost, or poor distribution networks. requires fewer processing steps, thus warranting future consideration as a While the majority of groundwater fluoride primary adsorbent of choice for fluoride removal from water. contamination is due to natural causes, anthropogenic sources of fluoride are also prevalent. Fluoride forms a common Dr. A. Z. Haddad, Prof. A. J. Gadgil aqueous effluent resultant in many chemical industries such as [8–10] Energy Technologies Area semiconductor, ceramic, glass, and metal-processing. Excess Lawrence Berkeley National Laboratory fluorides from these effluents pose significant health hazards, Berkeley, CA 94720, USA and underscore the need for cost-effective industrial wastewater E-mail: [email protected] treatment methods. Dr. C. D. Pilgrim Fluoride’s chemical affinity for aluminum makes adsorp- Department of Chemistry [11] University of California tion the most attractive option for remediation. The current Davis, CA 95616, USA state-of-the-art adsorbent is activated alumina (AA), a refined [12,13] Dr. A. M. Sawvel aluminum product with porous γ-Al2O3 crystal structure. Materials Science Division Recently, we and others have explored the use of its precursor, Lawrence Livermore National Laboratory bauxite ore, as an alternative adsorbent.[14–24] Bauxite is a globally Livermore, CA 94550, USA abundant ore of aluminum and is vastly less expensive. Whereas Dr. J. N. Hohman [22,25] The Molecular Foundry AA is about $2000 per ton, bauxite is less than $40. One Lawrence Berkeley National Laboratory advantage of activated alumina is its high porosity and surface Berkeley, CA 94720, USA area in excess of 200 m2 g−1.[12] Recently, we showed that treat- Prof. A. J. Gadgil ment of raw Indian bauxite at 300 °C results in a 15-fold increase Department of Civil and Environmental Engineering in specific surface area, from 11 to 170 2m g−1.[23] Adsorption on University of California this material was optimal at pH 6, and presumably the slightly Berkeley, CA 94720-1710, USA acidic environment provides an enhanced attraction between The ORCID identification number(s) for the author(s) of this article [14] can be found under https://doi.org/10.1002/adsu.201900005. hydroxyl sites on the bauxite surface and the anionic fluoride, although there has been no systematic study of the coordination environment of the bound fluoride. The enhancement of fluoride binding at mildly acidic con- ditions led us to test whether acid treatment of a thermally activated bauxite (TAB) might further enhance its adsorptive 2.1. Overview of Chemical Formation of ATAB properties.[1–7,11,14–23,25–27] In this study, we demonstrate that a combined thermal and acid treatment effectively converts Figure 1A shows the X-ray diffraction (XRD) pattern of TAB, as bauxite ore into a highly activated alumina. The resulting acid- well as ATAB, where samples were made from the same TAB tempered product is highly porous and exhibits fluoride batch. ATAB is shown before and after fluoride adsorption. adsorption capacities that are competitive with state-of-the-art Both patterns of ATAB are indistinguishable from one another, industrial materials. This potentially low-cost treatment pro- displaying peaks at 22 and 34 2θ(°)-λ- , assigned as boehmite. duces a high yield of boehmite, as demonstrated by powder X- Co Other smaller peaks at 48 and 54 2θ(°)-λ-Co are indexed as hem- ray diffraction (vide infra). We performed an investiga-tion of atite and anatase, respectively. The appearance of crystalline the performance and mechanism of acid-treated, thermally boehmite in ATAB XRD patterns is in stark contrast to the XRD activated bauxite (ATAB), including its synthetic preparation, pattern observed for TAB, which only shows crystalline features material characterization, a detailed kinetic and corresponding to hematite and anatase.[23] thermodynamic analysis, isotherm analysis, and exami-nation The treatment of various aluminum oxides and hydrox- of aluminum coordination environments. Fluoride adsorption ides with 5 M HCl facilitates the formation of crystalline onto ATAB follows pseudo-second order kinetics and exhibits a [29] −2 −1 boehmite. While crystalline phase changes are generally maximum second order rate constant of 6.57 × 10 g mg −1 observed under either extreme heat or pressure exposures, min at 25 °C. The chemisorption of fluoride onto ATAB is crystalline transformations in highly acidic solutions or under exothermic, spontaneous, and exergonic with a ΔG° of −5.02 kJ acidic-hydrothermal conditions (temperatures ranging from mol−1 and a H of 41.6 kJ mol−1 at 25 C using an initial Δ ° − ° 100 to 300 °C) have been observed previously in iron[30–34] and fluoride concentration of 10 ppm. aluminum[29,35] compounds, and are attributed to dissolution, precipitation mechanisms. In the case of bauxite, the thermal treatment at 300 °C facilitates the transformation of the initial 2. Results and Discussion aluminum phase in bauxite, gibbsite, to boehmite,[35,36] via dehydroxylation of gibbsite (loss of water molecules, and was Bauxite is a complex composite crystal comprised of various confirmed previously using thermogravimetric analysis-mass aluminum oxides, hydroxides, and additional compounds, [23] [28] spectrometry (TGA-MS)). Examination of Figure 1A reveals and they are variable in their composition across the Earth. no crystalline boehmite in TAB, rather existing only in the Scheme 1 shows the schematic overview of the conversion amorphous phase. This is rationalized by the dehydroxla- of bauxite to ATAB. This highly activated form of bauxite tion process that occurs when exposed to temperatures of effectively scavenges fluoride from aqueous solution under 300 °C, resulting in consecutive loss of water molecules from mildly acidic conditions. In the following sections, we will Al centers over extended periods of time; this prolonged expo- discuss our characterization of the various synthetic and sure degrades the edge-site packing environment, removing the physical aspects of the transformation of raw bauxite to ATAB, necessary hydroxyl groups, needed for the boehmite packing. and demonstrate its efficacy and absorption ability in later The Gibbs–Curie–Wulff law[29] supports this claim, telling that sections. linear crystal plane growth rates are proportional to the crystal surface energy densities. For boehmite, these surface energy densities are a direct consequence of the density of bound Scheme 1. Overview of the activation of crushed bauxite ore through heat and acid treatments, which converts bauxite into acid-treated thermally activated bauxite (ATAB). ATAB then readily adsorbs fluoride from aqueous solutions. in TAB to 401 m2 g−1 in ATAB. Post fluoride