Fly ash based geopolymer for heavy metal removal: A case study on copper removal

Highlights

• Highly amorphous coal fly ash based geopolymer has been synthesized.

• Geopolymer shows high efficiency for copper removal.

• The kinetic data fit well to the pseudo second order model suggesting chemisorption.

• According to the Langmuir isotherm, q_m was 2.38 mmol/g at a temperature of 45 °C.

Abstract

This work aims to evaluate the effectiveness of an amorphous geopolymer, synthesized from waste fly ash, as sorbent material for copper from aqueous solutions. The obtained geopolymer was found to be highly amorphous in structure due to the dissolution of fly ash glass phases. It has much higher removal efficiency compared to the raw fly ash. The removal efficiency was affected by solid/liquid ratio, temperature, reaction time, and Cu²⁺ initial concentration (C₀). The highest Cu²⁺ removal capacity was obtained at pH 6. The kinetic data were found fit to the pseudo second order kinetic model. Furthermore, it was found that Langmuir model is better than Freundlich model within the temperature range (25–45 °C). The maximum sorption capacity (q_m) occurred at 45 °C, reaching a value of 152 mg/g. The sorption process by the obtained geopolymer is endothermic in nature and more favorable at higher temperatures with enthalpy of adsorption of 39.49 kJ/mol. The activation energy of the sorption process at the optimum conditions was found to be 34.9 kJ/mol.

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 C_i, 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.