Biosorption of Zn(II) by live and dead cells of Streptomyces ciscaucasicus strain CCNWHX 72-14

Abstract

The biosorption characteristics of Zn(II) using live and dead cells of Streptomyces ciscaucasicus strain CCNWHX 72-14 as biosorbents have been investigated in the present research. Optimum conditions for biosorption were determined to be: pH adjusted to 5.0, agitated at 90 rpm and at a dose of 2 g/L. For initial zinc concentrations of 1–150 mg/L, batch biosorption data of live biomass preferred to be simulated with Freundlich model while those of dead strain fit Langmuir isotherm well. Experimental maximum biosorption capacity turned out to be 42.75 mg/g (0.654 mmol/g) for living material and 54 mg/g (0.826 mmol/g) for dead sorbents, respectively. The pseudo-second-order equation, instead of the pseudo-first-order one, was chosen to describe the time course biosorption process. In contrast to live biosorbents, dead biomass seemed to have lower binding strength with higher desorption efficiency at pH 1.0. Competitive biosorption revealed the order of competing metal ion to be: Cu²⁺ > Cd²⁺ > Ni⁺. FT-IR analysis indicated that more functional groups were involved in the biosorption process of dead adsorbents, compared with those linked to live biomass. Taken together, it can be concluded that dead cells of CCNWHX 72-14 were better and cheaper biosorbents than live ones.

Introduction

With tendency of accumulation in the food chain and extreme toxicity even in relatively low concentrations, heavy metals released into the environment are becoming a global concern [1], [2]. Zinc, which often exists in industrial effluents, is one of the 13 metals found in the contamination list proposed by the United States Environmental Protection Agency (USEPA) [3]. The World Health Organization (WHO) recommends a 5.0 mg/L maximum acceptable concentration of zinc in drinking water [4]. Consequently there is a significant interest regarding zinc removal from wastewater [5].

Conventional physiochemical methods for metals remediation include precipitation, filtration, coagulation, evaporation, ion exchange, membrane separation and solvent extraction. However, application of such processes is always expensive and ineffective in terms of energy and chemical products consumption, especially at low metal concentrations of 1–100 mg/L [6]. Therefore, there is a great need for an alternative technique, which is both economical and efficient. Biosorption, based on live or dead biosorbents, has been regarded as a cost-effective biotechnology for the treatment of complex wastewater containing heavy metals at high volume and low concentration [7]. Biosorption can be defined as the property of certain biomass to bind and concentrate selected ions or other molecules from aqueous solutions [8]. A number of economic biological materials, such as fungi, bacteria, yeast and algae, have been used to eliminate heavy metals from contaminated water [9], [10], [11].

In literatures, more and more Streptomyces strains have been employed as inexpensive biosorbents in the removal of metal ions. The simultaneous biosorption of Cu²⁺, Zn²⁺ and Cr⁶⁺ from wastewater by Streptomyces rimosus biomass was reported by Chergui et al. [12]. In another study, Yuan et al. [13] investigated the comparative biosorption of cadmium by two different Streptomyces strains K33 and HL-12. In recent research, dead Streptomyces strains were also widely used in the biosorption of metal ions, especially in a series of studies of Selatnia et al. [14], [15] and Nacera et al. [16]. However, little work has been done to compare the biosorption of Zn(II) using live and dead cells of given strain.

Streptomyces ciscaucasicus CCNWHX 72-14 has been shown to have higher degree of resistance to 13 mmol/L Zn²⁺, 0.8 mmol/L Cu²⁺, 0.6 mmol/L Cd²⁺ and 0.6 mmol/L Ni⁺ than other isolates. The 16S rRNA gene sequence of CCNWHX 72-14 was amplified with primers P1 and P6, from which the phylogenic tree was constructed [17]. As revealed in Fig. 1, the strain CCNWHX 72-14 can be classified in the branch of Streptomyces genera, being 100% identical to the S. ciscaucasicus strain HBUM 82686. Its recti-flexible aerial chains, composed of crinkly and columnar spores observed with scanning electron microscope, were illustrated in Fig. 2 [18]. The tremendous specific surface and capacious intracellular space produced by the strain, might play an important role in the removal of metal ions [19].

The objective of the present work is to investigate the biosorption potential of live and dead cells of S. ciscaucasicus CCNWHX 72-14 in the removal of Zn(II) from aqueous solution. The influence of different parameters on zinc biosorption, such as pH, agitation speed, biosorbent dose, initial metal concentration and contact time, were performed. Different equilibrium and kinetic models were applied to describe the biosorption process of live and dead biomass. The desorption experiments were implemented to identify the release of metal ions and recovery of biosorbents, while competitive biosorption and the Fourier transform infrared (FT-IR) analysis were also used in this study to look at potential binding sites and possible functional groups of live and dead biomass.