Metal(loid) Bioremediation by Microbial Polymers: Comparison
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Please note this is a comparison between Version 2 by Sirius Huang and Version 1 by Ayansina Segun Ayangbenro.

Environmental pollution arising from metal(loid)s is a result of industrialization, and has led to serious health issues. Conventional methods of metal(loid) removal often result in generation of secondary waste which is toxic to the environment. Bioremediation in combination with physicochemical techniques offer an excellent and effective means of removal. The use of secondary metabolites and extracellular polymers produced by microorganisms is an effective procedure employed in metal(loid) sequestration and reduction in toxicity of contaminated environments. These biopolymers have different chemical structures and have shown varied selectivity to different metal(loid)s. 

 

  • biodegradation
  • biofilms
  • bioflocculant
  • biopolymers
  • biosurfactant
  • secondary metabolites
  • pollution

1. Introduction

Environmental pollution due to swift growth in chemical industries, battery production, electroplating and steel industries, fertilizer and pesticide production, and mining activities is one of the most important challenges that are of human health concern. Pollution caused by metal(loid)s poses a health risk to living organisms. The accumulation of metal(loid)s in the environment as a result of natural and anthropogenic activities has significantly altered the nature of the environment [1,2][1][2]. In the midst of diverse metal ions, cadmium, chromium, lead and mercury in wastewater and soil are hazardous to the environment and have been linked to certain ailments such as birth defects, damage of internal organs such as kidney and liver, certain learning disabilities, and a host of other diseases. Due to their carcinogenic, genotoxic, mutagenic or toxic effects, their presence in the environment poses health risks and harmful effects on living organisms [3,4,5,6][3][4][5][6].
Metal(loid)s are natural components of the earth’s crust and are also produced from anthropogenic sources. Some metal(loid)s act as essential micro nutrients for living organisms, but at higher concentrations they can cause severe poisoning. Metal(loid) ions are non-degradable and thus persist in the ecosystem. They become bioavailable for removal as there is decrease in the surface tension of water at polluted sites [7].
Metal(loid) removal from contaminated environments usually involves three technologies namely: (1) Chemical, (2) physicochemical, and (3) biological techniques [2,7][2][7]. The techniques can overlap in some cases as a result of multidisciplinary research and paradigms shift in technology. However, there can be limitations to a particular technique as the operation condition might not be effective in a particular environment [2].
Conventional techniques for metal(loid) removal involves treatment of polluted soil with surfactants, organic and inorganic acids, water, reverse osmosis, adsorption processes, and metal(loid) chelating agents. However, these physicochemical techniques are expensive and do not guarantee effective removal of metal(loid) ions from the soil especially when metal(loid) concentrations are less than 100 mg/L. Other disadvantages include non-specificity of the techniques for metal(loid) binding, high energy demand, land space requirements, and sometimes the methods are impracticable and often exchange one problem for the other [4,8][4][8]. Effective sequestration of metal(loid) from polluted environments demands a good metal(loid) complexing agent. Such a complexing agent must possess characteristics such as environmental stability, high solubility and good metal(loid) complexation potentials [4].
Bioremediation is a cost-effective alternative for metal(loid) decontamination of polluted environments. It offers a long-term recovery and/or removal of metal(loid) ions from contaminated environments and can be used in combination with other techniques [2]. Various strategies have been developed by microbes for their continued existence in metal(loid) polluted environments. They adopt diverse detoxifying mechanisms that include bioaccumulation, biomineralization, biosorption and biotransformation for their survival [9] (Figure 1). These mechanisms are often triggered and activated when the concentration of metal(loid)s in their environment increases. They secrete many kinds of metal(loid)-binding metabolites, ranging from simple organic acids, alcohols to extracellular polymeric substances (EPS), slimes and sheaths, humic and fulvic acids, capsules, and biofilms which help in metal(loid) uptake from solution [10,11][10][11]. Metal sulfides and oxides are also trapped and absorbed by these extracellular polymers and proteins produced by microorganisms [11].
Figure 1.
Mechanism of metal(loid) sequestration.

2. Bioremediation by Microbial Polymers

Microbial products have generated increasing attention lately due to their biodegradable nature, diversity, and low toxicity. This makes them superior to their chemical counterparts. These metabolites provide defensive mechanisms, increase metal(loid) bioavailability and facilitate reproductive processes [12,13][12][13]. The metabolic pathways of these microorganisms have been exploited for degradation and removal of pollutants in the environments. In contrast to organic pollutants, metal(loid) contamination presents a difficult challenge; metal(loid) ions are not biodegradable but can be converted to base metal(loid), complexed with an organic ligand, methylated, precipitated or volatilized. An approach to metal(loid) removal involves stable complex formation between metal(loid) ion and microbial polymers. This is as a consequence of the electrostatic interaction between metal(loid) ions and negatively charged microbial polymers [14].
From the beginning of cellular life, microorganisms are periodically exposed to metal(loid)s. This is evident in many of their enzymes that are composed of different essential divalent and transition metals at their active sites. These essential metal(loid)s are involved in maintaining protein structure and catalyzing key enzymatic reactions. They are thus needed in small quantity for cellular metabolism. The uptake of these metal(loid)s is regulated via the homeostatic mechanisms that ensure sufficient, but not excessive intake [12]. However, other metal(loid)s have no known biological function, but instead, they damage, inactivate or block enzymes functions because of their affinity for the sulfhydryl groups of proteins. Resistance to metal(loid)s most likely evolved shortly after life started, in an early metal(loid)-polluted world [12]. The synthesis of microbial polymers is driven by the need to survive in hostile environment and to assist in neutralization of toxic elements by sequestration [15].
Microbial processes that facilitate the detoxification and metal(loid) mobility using resistant microbial strains can be used for removal of metal(loid)s from soil, effluents and wastewaters. The use of microbial polymers is evolving as a promising technique to improve and enhance the effectiveness of metal(loid) removal. These compounds are often economic, versatile, environmentally friendly and can reduce the concentration and toxicity of pollutants.
In addition, these biopolymers have the capability to survive a wide range of temperature and pH and can affect interfaces. pH affects the functional groups involved in metal(loid) binding—therefore, providing varying binding sites on the surface of the organisms at different pH values. At high pH, there are less H+ to compete with metal(loid)s for binding sites on the biopolymers. At neutral pH, they have functional groups that are negatively-charged and are capable of forming stable organo-metallic complexes with metal cations by means of diverse interactions [16]. A high temperature favors biopolymer-metal adsorption [17]. As the temperature increases, the affinity for the binding sites increases which favors direct contact between the biopolymer functional groups and metal ions [17]. However, a very high temperature can cause structural damage of the biopolymer and some functional groups, which can reduce the efficiency of the polymer [16]. The metal binding ability of these biopolymers depends on the nature of biomass, specificity, affinity, pH, temperature, hydrodynamics and functional groups present [16,18][16][18].
The nature of the biopolymer varies depending on the origin, extraction procedure and adsorption characteristics [16]. Guibaud, et al. [19] made a comparison of the origin of some biopolymers. Their results showed that biopolymers from activated sludge have higher affinity for Cd2+ and Pb2+ than polymers from pure strains of bacteria. Polymers from anaerobic granular sludge were found to have lower affinity and proton exchange capability [20]. The functional groups present on these biopolymers also vary in their affinity for metal(loid) ions [16]. The carboxyl and the phosphate groups are the main functional groups that are involved in binding of metal ions and are effective at neutral pH. Other functional groups include the amides, hydroxyl groups and nucleic acids [21]. These groups showed weaker binding ability compared to carboxyl and phosphate groups due to their high pKa values [22].
Microbial polymers are usually the first line of defense against metal(loid)s, and are important in protecting the interior of the microbial cell [16]. The use of isolated biopolymers is desired for easy availability in metal(loid) removal and circumvention of pathogenicity concerns of some producing organisms.

 

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