Prospects of Biogenic Xanthan and Gellan in Removal of Heavy Metals from Contaminated Waters

Abstract

Biosorption is considered an effective technique for the treatment of heavy-metal-bearing wastewaters. In recent years, various biogenic products, including native and functionalized biopolymers, have been successfully employed in technologies aiming for the environmentally sustainable immobilization and removal of heavy metals at contaminated sites, including two commercially available heteropolysaccharides—xanthan and gellan. As biodegradable and non-toxic fermentation products, xanthan and gellan have been successfully tested in various remediation techniques. Here, to highlight their prospects as green adsorbents for water decontamination, we have reviewed their biosynthesis machinery and chemical properties that are linked to their sorptive interactions, as well as their actual performance in the remediation of heavy metal contaminated waters. Their sorptive performance in native and modified forms is promising; thus, both xanthan and gellan are emerging as new green-based materials for the cost-effective and efficient remediation of heavy metal-contaminated waters.

Exopolysaccharides Producing Bacteria: A Review

“…Xanthan is synthesized through a Wzy-dependent pathway, which comprises several steps including the synthesis of exopolysaccharide precursors (nucleotide sugars GDP-mannose, UDP-glucose, and UDP-glucuronic acid), repeat-unit assembly on a lipid carrier located at the cytoplasmic membrane, membrane translocation to the periplasmic face, polymerization by a block-transfer mechanism involving Wzy polymerase, and export [ 312 ]. The building of pentasaccharide units is under the control of the gum cluster comprising 12 genes involved in the pentasaccharide repeating unit assembly, its decoration with substituents, polymerization, translocation, and secretion [ 312 ]. However, the key factors for the xanthan’s precursors’ biosynthesis are the genes xanA and xanB , which are not included in the gum cluster.…”

“…Specifically, the moisture absorption capability of these hydrogels was above 1000%, even after multiple dewatering/rehydration cycles. A recent review by Balíková et al (2022) highlighted xanthan prospects as green adsorbents for water decontamination [ 312 ]. A number of scientists have shown the functional groups of xanthan to be able to bind heavy metals from aqueous solutions and effectively remove them.…”

Production of Bacterial Exopolysaccharides: Xanthan and Bacterial Cellulose

“…In the past few decades, extensive work has been carried out to characterize BC biosynthesis [83][84][85][86][87][88]. Xanthan is produced using a Wzy-dependent pathway which has several steps, including the synthesis of exopolysaccharide precursors, repeat-unit assembly on a lipidcarrier located at the cytoplasmic membrane, membrane translocation to the periplasmic face, polymerization by a block-transfer mechanism involving Wzy polymerase, and export (Figure 1D) [36].…”

“…Homopolysaccharides are commonly synthesized using synthase and extracellular synthesis pathways, while heteropolysaccharides are produced by the Wzx/Wzy-dependent pathway and the ABC transporter-dependent pathway. For example, heteropolysaccharide xanthan is synthesized through a Wzydependent pathway and homopolysaccharide BC through the synthase pathway [35][36][37]. The biosynthesis of xanthan and BC begins with the synthesis of exopolysaccharide precursors UDP-glucose for BC and UDP-glucose, GDP-mannose, and UDP-glucuronic acid for xanthan.…”

“…Bacterial EPSs have many unique beneficial properties such as biocompatibility, biodegradability, non-toxicity, a high degree of polymerization, the ability for gelation, high adhesive ability, viscoelasticity, pseudo-plasticity, a thixotropic nature, renewable sourcing, and easy modification. In addition, some bacterial EPSs also have extensive bioactivities, including antibacterial, antifungal, antiviral, antioxidant, anti-inflammatory, antitumor, antidiabetic, antiulcer, anticoagulant, immunomodulatory, prebiotic, wound healing, and cholesterol-lowering activities [1,3,25,27,[35][36][37][38][39][40][41][42][43]. Therefore, they have extensive commercial applications in biomedicine, food, pharmaceuticals, cosmetics, electronics, environmental remediation, and the oil and gas industries [1][2][3][4][5][6][7][8][9].…”