RESUMO
Nano- and micron-sized catalysts are continuously being discovered as efficient tools for pollutant oxidation. Their small size motivates their entrapment in beads or capsules for easier handling, but this is normally followed by reduced reaction kinetics due to slower mass transfer within the encapsulation matrix. In this study, liquid-core encapsulation was explored as a way to overcome this limitation. Biogenic manganese oxides (BioMnOx) were chosen as representative catalysts of interest, and two organic pollutants, glyphosate and bisphenol A, were used as model substrates. Different capsule compositions were examined to ensure rapid diffusion with high preservation of oxides and the oxide-forming bacteria. Glyphosate oxidation was found to follow the reported behavior of abiotic birnessite and was highly dependent on pH and oxide concentration. Thanks to the strong relationship between oxidation kinetics and oxide levels, the BioMnOx localized inside the capsules removed glyphosate significantly faster than suspended oxides, and their reuse for several treatment cycles was demonstrated. Bisphenol A, which is more sensitive to diffusion rates than to oxide concentrations, was removed by encapsulated BioMnOx at nearly the same speed as in suspension. Such encapsulation allows simple separation and concentration of reactive surfaces and enables fast transport of substrates in and transformation products out of the capsule, making it a promising way to simplify the use of suspended catalysts at improved performance.
RESUMO
E. coli cells overexpressing the enzyme atrazine chlorohydrolase were coated using layer-by-layer self-assembly. The polymeric coating was designed to improve the surface properties of the cells and create positively charged, ecologically safe, bio-hybrid capsules that can efficiently degrade the herbicide atrazine in soils. The physio-chemical properties of the bacteria/polymer interface were studied as a function of the polymeric composition of the shell and its thickness. Characterization of cell viability, enzyme activity, morphology, and size of the bio-capsules was done using fluorescence spectroscopy, BET and zeta potential measurements and electron microscopy imaging. Out of several polyelectrolytes, the combination of polydiallyldimethylammonium chloride and polysodium 4-styrenesulfonate improved the surface properties and activity of the cells to the greatest extent. The resulting bio-hybrid capsules were stable, well-dispersed, with a net positive charge and a large surface area compared to the uncoated bacteria. These non-viable, bio-hybrid capsules also exhibited a kinetic advantage in comparison with uncoated cells. When added to soils, they exhibited continuous activity over a six-week period and atrazine concentrations declined by 84%. Thus, the concept of layer-by-layer coated bacteria is a promising avenue for the design of new and sustainable bioremediation and biocatalytic platforms.