Fly ash based geopolymer for heavy metal removal: A case study on copper removal
Introduction
Copper as a heavy metal is hazardous to human health and the environment [1]. It is widely used in industry and is accumulating in their waste streams. Adsorption of Cu2+ has short and long term effects on human health [1], [2], [3]. Many effective methods are used for copper removal from wastewater including chemical precipitation [4], ion exchange, reverse osmosis, electrochemical treatment, evaporative recovery, and adsorption [5].
Recently, natural materials with low cost treatment such as silica, mud, agricultural wastes, ash, and solid wastes were used as sorbent materials. Some researchers used palm fruit, activated carbon prepared from palm oil empty fruit bunches [2], palm kernel shell based activated carbon [1], and oil palm leaf powders [6] for Cu2+ removal from aqueous solutions. Other plants such as Cassia angustifolia [7], Tridax procumbens [8], Pleurotus cornucopiae [9], Sorgum vulcaris dust [10], spruce sawdust [11], mangrove barks [12], polar and oak leaves [13] have been used for the same purpose. The conducted studies investigated the effects of different conditions (pH, contact time, and Ci, and adsorbent dosage) on adsorption capacity. Arivoli et al. [14] prepared activated carbon from solid waste called Terminalia catappa Linn shell and studied its effectiveness in adsorbing copper ion from aqueous solution. Fixed-bed tests were conducted to investigate the capabilities of a modified silica with respect to the selective removal of copper ions from multi-metal solutions [15]. Sthiannopkao and Sreesai [16] used lime mud and recovered boiler ash to remove the heavy metals from metal finishing wastewater through sorption and precipitation process. A high removal efficiency of Cu2+ (99%) and lower leachability were obtained. Fly ash based zeolites for the removal of heavy metal ions were used by Solanki et al. [17]; the synthesized zeolite obtained has high removal efficiency of Cu2+ up to 100%. In addition to the synthetic zeolites, Gabai et al. [18] tested the adsorption capability of chelating resins and activated carbons for Cu2+ removal.
There is a significant trend in recycling of waste materials and converting it to usable and valuable materials. One of these waste materials is coal fly ash. The disposal of the large amount of fly ash has become a serious environmental and economic problem [19], [20]. One approach to deal with fly ash waste is to convert it to geopolymer, which is not only effective for heavy metals removal, but also helps in solving the problem of ash accumulation as an industrial waste product. Geopolymer is amorphous material produced by reacting solid aluminosilicate with highly alkali hydroxide. It consists of a polymeric silicon–oxygen–aluminum framework with alternating silicon and aluminum tetrahedral joined together in three directions by sharing all the oxygen atoms [21]. Geopolymers have been synthesized from fly ash [19], natural zeolite [22], kaolinite [23] and volcanic ash [24]. They have been used as adsorbent materials for removal of Cd, Ni, Pb(II), Cu(II), phosphate, NOx, boron, fluoride, radionuclide of 137Cs and 90Sr, as well as dyes [19].
The aim of this research is to determine the capability of fly ash based geopolymer in removing copper from aqueous solution as compared to other sorbents. Fly ash was characterized before and after geopolymerization by X-ray diffraction (XRD) and X-ray fluorescence (XRF). The behavior of sorption process under different conditions (solid/liquid ratio, copper concentration, pH, temperature, and contact time) was investigated. Langmuir and Freundlich models were used to study copper adsorption isotherm on the produced geopolymer. The thermodynamic parameters of the adsorption process were also evaluated.
Section snippets
Materials
Coal fly ash samples were collected from Rajhi Cement Plant, Jordan. Fly ash was homogenized by grinding using pestle and mortar. After sieving the fraction of less than 45 μm was used to prepare the geopolymer. The synthesized geopolymer was then crushed and ground to 100% less than 200 μm [25]. The chemicals used in this study were of reagent grade. Copper solutions with different concentrations were prepared from 1000 ppm standard (Cu2+) solution (Merk®).
Instrumentation
X-ray diffraction (XRD) analysis was
XRD analysis
The XRD pattern of both fly ash and the synthesized geopolymer is shown in Fig. 1. It was found that coal fly ash sample contains the following major phases: quartz, hematite and mullite. Cristobolite and plagioclase minerals were found in trace quantities. The major part of the ash samples has an amorphous structure; such amorphousity makes fly ash reactive material [30].
After geopolymerization process, the resulting geopolymer has lost almost all crystalline phases present in the initial fly
Conclusions
The present work focused on the synthesis of fly ash—based geopolymer and its adsorption characteristics toward Cu2+ removal. The obtained geopolymer was found to be highly amorphous. It was also found that the adsorption capacity of the synthesized geopolymer is high compared to other sorbents used in previous work. From the economical and environmental point of view, this application provides an adequate method to solve the problem of accumulation of fly ash waste material. Therefore, it is
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