Coal Fly Ash-derived Mesoporous Calcium-Silicate Material (MCSM) for the Efficient Removal of Cd(II), Cr(III), Ni(II) and Pb(II) from Acidic Solutions

AbstractEnvironmental pollution by heavy metal ions has received worldwide attention due to their long-term toxicity, non-biodegradability and accumulation in living organisms. Mesoporous calcium-silicate material (MCSM), a cost-effective sorbent synthesized from the desilication liquor of coal fly ash, has been tested its potential for the removal of Cd(II), Cr(III), Ni(II) and Pb(II) from acidic solutions. The effect of initial concentration, m/V ratios, solution pH and ionic strength on the sorption was investigated, and the performance of the material on the removal of heavy metals from simulated wastewater was evaluated. At a m/V ratio of 2g L-1, the maximum sorption capacity of MCSM for metals follows the order: Cd(II)≈Ni(II) (5.52 mmol g-1) > Cr(III) (2.88 mmol g-1) > Pb(II) (2.17 mmol g-1). However, the capacity for Cr(III) significantly decreased with comparatively higher metal concentration due to the strong hydration of Cr(III) which destroyed the crystalline phase of material. The initial concentration had a limited positive effect on the sorption, and the removal efficiency of metals only maintained at a relatively higher level with m/V ratios of up to 2 – 5g L-1. At a low m/V ratio (0.75g L-1), sorption was significantly influenced by solution pH and competition cations under strongly acidic conditions (pHinitial= 2), and the adverse effect disappeared when solution pHinitial ≥3. At an m/V ratio of 5g L-1, the removal efficiency of metals maintained in the range of 96.2 – 100.0% varied with different heavy metal ions. Considerable uptake of Zn(II), Mn(II), Fe(III) and Cu(III) by MCSM was also observed in the simulated wastewater, and the 8 metal ions (each 0.001M) were efficiently removed and the remaining metal concentration < 0.1mg L-1 (except remaining Ni(II) concentration < 1.0mg L-1) at an m/V ratio of 20g L-1. The mechanism responsible for the sorption included surface complexation, ion exchange, and probably precipitation at a high m/V ratio (20g L-1). Ion exchange with Ca(II) in MCSM occurred when m/V ratios ≥ 2g L-1(42.1 – 89.6% for m/V = 2 – 5g L-1), whereas surface complexation would probably occur when m/V ratios less than 1g L-1 due to its structural damage by extensive dissolution to form crosslinked structure which resembles vitreous silica. The results suggest that MCSM has the potential for use as an effective and low-cost sorbent for removing hazardous metal ions from wastewater. Besides, MCSM is suitable for use as a substitute of cement and disposed by inexpensive solidification/stabilization technique after being used as a sorbent. Such a process is helpful for the simultaneous realization of the reutilization of coal fly ash and the removal of toxic metals from wastewater