Introduction
Mercury is considered as a highly toxic heavy metal posing serious threat to human health. The discharge of effluents containing mercury in soil, sediments, and water causes irreversible damage to the environment. The main sources of mercury emissions besides the natural origins are the ore mining, the burning of fossil fuels, and the industrial production processes. Heavy metal like mercury does not degrade and tend to accumulate in living organisms causing various disorders. Therefore, it is important to remove mercury, primarily from waste water, prior to disposal to the environment. Currently, there are a variety of technologies available for mercury removal, such as sulphide precipitation, coagulation, co-precipitation, and reverse osmosis. Although these techniques are effective to some extent, most of them have disadvantages such as high cost, operational difficulties, complicated industrial setups, and large toxic irreversible sludge. In this work we have developed a cheap and effective remedy for removal of mercury from aqueous solutions by adsorption onto alkali-activated fly ash.
Results and Discussions
Fly Ash is a by-product of coal combustion whose major components are oxides of silicon, aluminium, iron, calcium and magnesium. The oxides form a negative layer on the fly ash and hence act as an adsorbent to metal ions, especially mercury ions, through electrostatic attraction. Since fly ash can be obtained cheaply in large quantities, it is used as an effective adsorbent. However raw fly ash is not very potent as an adsorbent. Therefore, it is activated by treatment with alkalis in order to increase its adsorbent power. Activation leads to the increase in surface area and pore volume which facilitates adsorption.
Alkali Activation of Fly Ash
Raw fly ash was mixed with a 14M mixture of sodium and potassium hydroxide solution and heated in an oil bath at 85 °C for 48 hours. The mixture was filtered, washed with hot water and acetone, dried and stored in a desiccator prior to use.
Adsorption Experiment
The adsorption of mercury ions onto activated fly ash was studied by shaking solution of mercury ions having concentrations ranging from 10 ppm to 100 ppm with a fixed amount of adsorbent (20 mg) at 25 °C. The mixtures were shaken for 30 minutes and the concentration of mercury ions left after adsorption was determined by inductively coupled plasma-optical emission spectroscopy (ICP-OES).
Adsorption Studies
Effect of dosage:
In order to determine the optimum amount of adsorbent for maximum removal of mercury, varying amounts of adsorbent ranging from 5 mg to 50 mg were tested. The minimum amount of adsorbent required for maximum removal of mercury was found to be 20 mg.
Effect of pH:
Metal solutions were prepared at varying pH ranging from 4 to 10 and the optimum pH where fly ash showed maximum adsorptive capacity was found to be around 7.85.
Effect of metal ion concentration:
When metal ion concentration was varied from 10 ppm to 100 ppm, maximum removal of mercury was obtained from 30 ppm solution after which the adsorption capacity of fly ash decreases with increasing metal ion concentration.
Effect of contact time:
The effect of contact time of adsorbent on mercury ion concentration was conducted with 30 ppm metal ion concentration, adsorbent dose of 20mg, at pH 7.85, temperature 25 °C and contact time varying from 5 min to 140 min. About 80 percent removal of mercury ions was observed after about 100 min contact time.
Adsorption isotherm
Figures depicting a) Freundlich isotherm and b) pseudo second-order kinetics at pH 7.85
A. log qe = log KF + (log Ce)/n B. t/qt = 1/(k2q2e) + t/qe
Kinetic model was used to analyze the experimental data based on which the adsorption mechanism was found to follow pseudo second order kinetics with very high correlation coefficient. Moreover, it was found that the Freundlich isotherm fits well with the adsorption process. The details of the result is summarised in table 1.
Table 1: Kinetics and Freundlich parameters for mercury adsorption onto fly ash.
| Pseudo 2nd order kinetics | Freundlich Isotherm | ||
| qe(mg/g) | 29.25 | KF (L/mg) | 0.940 |
| k2 (g/mg min-1) | 0.0136 | 1/n | 0.98 |
| R2 | 0.981 | R2 | 0.975 |
Where qe is the amount of mercury ions adsorbed at equilibrium per unit weight of adsorbent, k2 is pseudo second-order constant, KF and n are Freundlich constants corresponding to adsorption capacity and intensity respectively
Conclusion
Activated fly ash is a cheap and highly effective adsorbent for removal of mercury ions from aqueous solutions. The maximum adsorption capacity of the activated fly ash for mercury ions was found to be ~30 mg/g which is quite good. The adsorption process is well supported by Freundlich adsorption isotherm and pseudo second order kinetic model. Moreover, the results suggest that the activated fly ash can be applied for the removal of mercury ions from waste water prior to its disposal.