The influence of equilibration conditions and hydroxyapatite physico-chemical properties onto retention of Cu2+ ions

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Abstract

The influence of equilibration conditions and hydroxyapatite (HAP) physico-chemical properties onto retention of Cu2+ ions was studied in batch conditions. The amount of cation removed from the solution increased with increasing pH, reaching almost 100% at pH 3, 4 and 7 for 5 × 10−4, 10−3 and 5 × 10−3 mol/dm3 solutions, respectively. Contact time necessary for reaching equilibrium was found to increase with the increase of Cu2+ concentration. Kinetic and equilibrium data were best described by pseudo-second-order kinetic model and Langmuir theoretical model. The calculated values of separation factors and Gibbs free energy change confirmed that the sorption was spontaneous and thermodynamically feasible at room temperature. The experiments conducted using HAP samples with different physico-chemical characteristics have revealed that the amounts of sorbed Cu2+ depended mainly on the specific surface area and crystallinity of the applied powders. Desorption of Cu2+ was more efficient in acidic conditions than in the solution of competing cation—Ca2+. The samples with higher sorption capacities also demonstrated higher stability; consequently, from the aspects of both higher sorption and lower desorption, utilization of low-crystalline HAP samples with high specific surface area was superior for immobilization of Cu2+ ions. Taking into account molar Cu/Ca ratios, observed final pH changes, copper speciation in the function of pH and the results of X-ray diffraction analyses, conclusions about sorption mechanisms at different experimental conditions were derived.

Introduction

Copper is a heavy metal widespread in the environment, with the average concentrations in the earth's crust and soil of 24–55 and 20–30 mg/kg, respectively [1]. However, due to various human activities, the amount of Cu2+ in the environment increases every year. The largest sources of copper pollution are metal mining and manufacturing, as well as petroleum refining, coal mining and manufacturing of cement, lime, concrete, paper, wood, glass and ceramic products.

In small concentrations this element is essential for living organisms. Furthermore copper deficiency may cause various effects on human health such as anemia, osteoporosis, decreased glucose tolerance, arthritis, cardiac arrhythmias and neurological problems [2]. On the other hand, excess copper in cells causes toxicity, as the redox properties, essential for its function in cuproenzymes, can also result in marked reactive oxygen species formation that can damage lipids, nucleic acids and proteins [3]. Therefore, Cu2+ containing effluents from various industries require special treatment before their discharge into receiving waters.

Sorption technologies, as a method for the removal of heavy metals from wastewaters, have gained much attention in the last decade due to their simplicity, low cost and high efficiency [4]. Various mineral sorbents have been evaluated as potential Cu2+ removal agents from aqueous media, such as clinoptilolite [5], bentonite [6], palygorskite [7], kaolinite [8], aluminum hydroxide and oxyhydroxides [9]. Furthermore, a potential application of various organic materials: activated carbon [10], peanut hull pellets [11], dried sunflower leaves [12], sugar beet pulp [13], Capsicum annuum seeds [14], etc., for Cu2+ removal has been considered as well.

Apatite materials of different origin (mineral, biological and synthetic) were fund to be potential sorbents for numerous heavy metals and radionuclides [15], [16]. Ion-exchange with Ca2+ from apatite crystal lattice, dissolution/precipitation and specific cation sorption were identified as main removal mechanisms, depending on the cation type. In comparison to other divalent, toxic, heavy metals such as Pb2+, Cd2+ and Zn2+, much less attention has been paid to Cu2+ immobilization by apatites in the scientific literature. The understanding of Cu2+ interaction with apatite is not only important for finding better ways to treat contaminated sites, but it is also useful for understanding the processes which occur at the phosphate rock–Cu2+ solution interface, and phenomena that take place when this cation interacts with bones and teeth which consist mainly of apatite [17]. Cu2+ sorption was studied using: carbonate hydroxyapatite derived from eggshell waste [18], bone char [19], spent animal bones [20], phosphate rocks [21] and synthetic powder [22]. Although apatite sorbents were, in general, evaluated as promising agents for Cu2+ removal from aqueous media, the comparison of sorption capacities and main operating sorption mechanisms is difficult due to the presence of various apatites and other solid phases in phosphate rocks and bone materials.

The understanding of the removal mechanism is fundamental in properly designing and operating sorption processes, therefore in this study synthetic calcium hydroxyapatite (Ca10(PO4)6(OH)2, HAP) samples were used as Cu2+ sorbents. The aims of this study were to investigate both the influence of equilibration conditions (contact time, pH and Cu2+ concentration) as well as HAP physico-chemical properties on Cu2+ sorption, in order to evaluate main sorption mechanisms at different experimental conditions, and to estimate the reversibility of Cu2+ immobilization by HAP using leaching tests. Presented results have potential application in optimization of HAP matrix, as well as equilibrium conditions, for Cu2+ immobilization.

Section snippets

HAP sorbents

As sorbents, synthetic HAP samples were used (denoted in this study as HAP1-8). They were prepared by neutralization of Ca(OH)2 with H3PO4 [23], at various temperatures, mixing speeds, concentrations of reactants, H3PO4 addition rates, and with/without inert atmosphere and precipitate aging, in order to produce powders with different physico-chemical characteristics. In addition, the commercial HAP (DNA grade Bio-Gel HTP) supplied by Bio Rad laboratory, Richmond, CA, was used (denoted as HAP9).

Influence of the sorption conditions

Physico-chemical properties of HAP samples

The major physico-chemical properties of HAP powders are listed in Table 2 [23].

The variation of the synthesis parameters caused variation in physico-chemical properties of obtained samples. The values of specific surface area (Sp) were in the range from 24 to 72 m2/g. As molar Ca/P ratio of 1.67 is characteristic for pure stoichiometric HAP, results from Table 2, indicated that all samples, except HAP4 and HAP8, were calcium deficient. The fractions of crystalline phase (Xc) ranged from 0.19 to

Conclusions

Sorption of Cu2+ ions by HAP depended on equilibration conditions as well as on HAP properties. Presented experimental results have shown that:

  • 1.

    Sorbed amount of Cu2+ generally increased with the increase of pH reaching near 100% at pH 3, 4 and 7 for 5 × 10−4, 10−3 and 5 × 10−3 mol/dm3 solutions, respectively. The pHPZC value of HAP sorbent decreased with the increase of initial Cu2+ concentration.

  • 2.

    Contact time necessary for reaching sorption equilibrium increased with the increase of initial Cu2+

Acknowledgements

This work was supported by the Ministry of Science and Environmental Protection of the Republic of Serbia, under Project No. 142050G.

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