Characterization of metal removal of immobilized Bacillus strain CR-7 biomass from aqueous solutions

Abstract

Bacillus strain CR-7 of multiple metal and antibiotic resistances was isolated. Its metal adsorption under different pretreatments and immobilizations from aqueous solution was characterized. Pretreatment with NaOH (0.1 mol L⁻¹) significantly improved Cu²⁺ adsorption capacity of the bacterial biomass. Sodium alginate (2%) was the ideal immobilization matrix. The immobilized and pretreated biomass had an obvious “orderliness”, following the order of Cu²⁺ > Zn²⁺ in the solution containing these two metals, and following the order of Pb²⁺ > Al³⁺ > Cr⁶⁺ > Cu²⁺ > Fe³⁺ > Zn²⁺ = Ni²⁺ > Cd²⁺ = Co²⁺ > Mn²⁺ in the solution containing these 10 metals. ΔH° and ΔS° of Cu²⁺ adsorption were +7.68 J/mol and +16.628 J/mol K, respectively. The infrared peak of –N–H shifted greatly after Cu²⁺ adsorption. After adsorption treatment, some molecular groups disappeared in un-immobilized biomass but were still present in the immobilized biomass. Cu²⁺ adsorption fit both Langmuir and Freundlich isotherm models. It was concluded (1) that the Cu²⁺ adsorption process was endothermic, (2) that –N–H is a most important Cu²⁺-binding group, (3) that immobilization prevents loss or damage of the Cu²⁺-binding molecular groups, and (4) that Cu²⁺ adsorption of pretreated and immobilized biomass is homogeneous.

Introduction

Growing amount of heavy metal-polluted wastewater is rooted in the aggressive industrialization and urbanization [1]. If no further treatment, the heavy metals in the wastewater are likely eventually absorbed by and accumulated in living organisms, and threaten health of the living organisms. Even if some trace elements such as copper are essential to growth and development of the living organisms, they have toxic effects on the living organisms at high concentrations [2]. The polluted wastewater has therefore received much concern [3]. Methods on treatment of heavy metal-polluted wastewater can be divided into (1) physical and/or chemical reactions such as chemical precipitation, ion exchange, filtration, and (2) adsorption either by biomaterials such as microbial and plant derived biomasses [4], [5], [6] or by non-biomaterials such as fly ash, carbon slurry, red mud, kaolinite, baggage fly ash, bentonite, electric furnace slag, and montmorillonite [7], [8], [9], [10]. Physical and/or chemical reactions are not fast for removal processes but easy to commercialization and application. However, physical and/or chemical reactions-based methods have unacceptable defects such as high costs, high energy consumption, secondary pollution, and/or production of a large amount of toxic chemical sludge which is difficult to treat [5], [11], [12]. Relatively, adsorption methods have an advantage over physical and/or chemical methods because of low cost and low energy, and particularly due to no the secondary pollution [4], [13]. However, if no other auxiliary measures, the microbial biomass-based treatment methods are hardly applied because of several major defects such as solid–liquid separation problems, possible biomass swelling, inability to regenerate/reuse, use in the continuous mode, and development of high pressure drop in the column mode [14], [15]. To overcome these defects, immobilization techniques for microbial biomasses have been developed [14], [16].

Most researches on immobilization techniques-based metal removal from the solutions that have been conducted were based on the use of granulized microbial adsorbents packed in columns [16]. This approach seems to be inapplicable to metal removal by free cells because the resulting bed is easily plugged by free cells [16]. Therefore, microbial immobilization techniques still needs further study. Toward this goal, choice of the immobilization matrices is a key step. By now, numbers of immobilization matrices have been developed and used, such as sodium or calcium alginate, polysulfone, polyacrylamide, polyurethane and silica [14]. Even so, immobilization techniques specific to bacterial species are required not only because of bacterial species diversity [17] but also owing to difference in the nature of immobilization matrices.

Although Bacillus biomass has been used for removal of heavy metal from aqueous solution [14], [16] researches on heavy metal removal by immobilized Bacillus biomass from the solutions were very limited. Immobilization matrices used in Bacillus biomass included Diaion SP-850 resin [18], silica gel [21], Amberlite XAD-4 [22], and calcium alginate [23]. However, little is known about characteristics of heavy metal removal by Bacillus biomass from aqueous solution with mixed metals, and effects of immobilization on metal-binding molecular groups of the biomass. In this study, Bacillus strain CR-7 was chosen as an adsorbent because of its resistance to multiple metals. The aim of this study was to characterize metal adsorption of the bacterial biomass immobilized with sodium alginate, gelatin, and polyvinyl alcohol (PVA).