Adsorption behavior of Zn(II) on calcinated Chinese loess

https://doi.org/10.1016/j.jhazmat.2008.04.059Get rights and content

Abstract

Chinese loess has proven to be effective in removing Zn(II) from aqueous solutions, but the resultant adsorbent–water slurry is difficult to separate. In this paper, the crude loess was calcinated to improve the separation efficiency of slurries in terms of sedimentary rate by increasing the particle sizes of the adsorbent. The sorption capacities of different sorbents, including crude loess, calcinated loess, de-organic crude loess and acid-treated calcinated loess, were obtained and sequenced. The adsorption capacity of the calcinated loess towards Zn(II) was found to be as high as 113.6 mg g−1. The adsorption isotherms and kinetics of calcinated loess were best-fit with the Freundlich isotherm and the pseudo-second order kinetics, respectively. The thermodynamic analysis revealed that the adsorption was exothermic and spontaneous with a high preference for Zn(II) removal. The adsorption of Zn(II) on calcinated loess implies an ion exchange of the solute with calcite and goethite due to the observed FT-IR and XRD patterns as well as the predicted mean free energies (−11.58 to −9.28 kJ mol−1 by D–R model). The byproduct of adsorption can be purified and refreshed by using a 0.01 M HCl solution.

Introduction

Zinc is one of the main pollutants in acid mining drainages. In developing countries, metal mining and metallurgy industrial departments produce large quantities of wastewater containing high concentration of Zn(II) [1], [2]. Considering the toxicity of Zn(II), treatment methods have been widely developed. Chemical precipitation is the most widely adopted method. Due to the low dissolution coefficient of zinc sulfide, sodium sulfide or hydrogen sulfide is always used to remove ionic zinc from aqueous solution to extremely low or nondetectable levels [3], [4]. Chemical precipitation can also be obtained by adjusting the solution pH to alkaline states which will lead to the precipitation of Zn(II) [5]. Nevertheless, the costs of chemical additives are high and the salinity of the wastewater will be increased, which limits the applicability of this method.

Fixation of Zn(II) through ion exchange is also an optional treatment method. A selective elimination is possible by means of heavy metal-selective ion exchangers which exhibit a high affinity for heavy metals. However, this method is not extensively adapted to metals removal in industrial wastewater treatment [6].

Adsorption can be used to attain a similar target. Many sorbents have been developed and are being continually tested. Minerals such as apatite and goethite have been reported with excellent purifying capabilities for metal cations. Industrial wastes such as activated sludge, coal fly ash and iron slag have also been studied for the applicability for Zn(II) removal. Natural products such as leaf [7], coca shell [8] and sea weed [9], can be used to remove heavy metals by producing activated carbons through further processing. Several kinds of natural soils have been observed to have high adsorption capacity for Zn(II) and optimum conditions have also been extensively investigated and reported [10], [11], [12].

Chinese loess is widely distributed throughout western China and has been studied as an adsorbent for Zn(II) removals from aqueous solution in the authors laboratory. The intrinsically abundant constituent in loess is calcite. Calcite has been found to play a major role in Zn(II) adsorption [13], [14]. The adsorption capacity is rather high in comparison with other adsorbents, showing potential utilizability in practice. In western China, the acid mining drainage is a major source of contamination to the environment. It is particularly beneficial to investigate the applicability of locally prolific loess for wastewater treatment. Since the main composition of loess is clay and silt [15], the treatment of Zn(II) in solution produces mixtures with a large amount of fine solids that are difficult to filter or precipitate. The separation of the sorbent after equilibrating with Zn(II) solution becomes an intractable engineering problem.

The main objectives of this paper are to develop a method to improve the separating properties of loess after adsorption with Zn(II) and to investigate the adsorbing behavior as well as the intrinsic uptake mechanism. Firstly, the improvement of the sedimentary rate of the adsorbent requires an increased particle size of the sorbent that could be achieved after calcination at moderately high temperatures. Secondly, the Zn(II) adsorption behavior of the obtained sorbent was investigated and compared with those of the crude loess without any treatment. Finally, several factors, including solution pH, slurry concentration, reaction duration and temperature, which may affect the adsorption behavior of Zn(II) on the sorbent, were studied. The treatment method for the sorbent adsorbed with Zn(II) was also studied for possible recycling utilization.

Section snippets

Preparation of the adsorbent

Crude loess (CL) was taken from suburban area of Xi’an, China. The soil was put into a clean ceramic cup and oven calcinated at 650 °C for 7 h [16]. The calcinated loess (CAL) was ground to a powder with a ceramic mortar and then stored in a plastic bag after being cooled down to room temperature. The mortar was previously cleaned three times with deionized water (DW) and then polished with ethanol to avoid contamination of the samples. The crude soil and the acid-treated calcinated soil (ATCAL)

Characterization of the sorbent

Table 1 shows the grain size distribution of the adsorbents. The clay component decreases from 20.5% of CL to 6.1% of the CAL. After calcination, the mass percentages of sand and silt particles increase from 14.1 and 65.4 in CL to 15.6 and 78.3 in CAL, respectively. And the specific surface area of the adsorbents is found to decrease from 24.1 to 20.7 m2 g−1. It is obvious that the particle sizes are greatly increased by the calcination of CL. Other basic physicochemical parameters, including the

Conclusions

  • (1)

    Calcinations of crude loess proved to be effective in improving the separation efficiency for slurries and to increase the particle sizes of the sorbent.

  • (2)

    The Zn(II) adsorption amount on CAL increases steadily with increasing equilibrium pH and with decreasing slurry concentration and temperatures.

  • (3)

    The sorption capacities of different sorbents are sequenced as CL > CA L > DCL > ATCAL. The adsorption on CAL is multilayered, exothermic and spontaneous and the isotherm follows the Freundlich model. The

Acknowledgments

The authors would like to express their sincere gratitude to the Key Project of National Natural Science Foundation of China (NSFC) (Grant 50538080) and National Science Fund for Distinguished Young Scholars (Grant 50425825) for the financial support to this study. The helps from Professor D.G. Fredlund in proof checking of this paper and the comments from two anonymous reviewers are also appreciated.

References (32)

Cited by (0)

View full text