Optimization of Carbon Dioxide and Valeric Acid Utilization for Polyhydroxyalkanoates Synthesis by Cupriavidus necator

The utilization of captured CO₂ as a part of the CO₂ capture and storage system to produce biopolymers could address current environmental issues such as global warming and depletion of resources. In this study, the effect of feeding strategies of CO₂ and valeric acid on cell growth and synthesis of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) [P(3HB-co-3HV)] in Cupriavidus necator was investigated to determine the optimal conditions for microbial growth and biopolymer accumulation. Among the studied CO₂ concentrations (1–20 %), microbial growth and poly(3-hydroxybutyrate) accumulation were optimal at 1 % CO₂ using a gas mixture at H₂:O₂:N₂ = 7:1:91 % (v/v). When valeric acid was fed together with 1 % CO₂, (R)-3-hydroxyvalerate synthesis increased with increasing valeric acid concentration up to 0.1 %, but (R)-3-hydroxybutyrate synthesis was inhibited at >0.05 % valeric acid. Sequential addition of valeric acid (0.05 % at Day 0 followed by 0.025 % at Day 2) showed an increase in 3HV fraction without inhibitory effects on 3HB synthesis during 4 d accumulation period. The resulting P(3HB-co-3HV) with 17–32 mol  % of 3HV is likely to be biocompatible. The optimal concentrations and feeding strategies of CO₂ and valeric acid determined in this study for microbial P(3HB-co-3HV) synthesis can be used to produce biocompatible P(3HB-co-3HV).     

Increased 3HV Concentration in the Bacterial Production of 3-Hydroxybutyrate (3HB) and 3-Hydroxyvalerate (3HV) Copolymer with Acid-Digested Rice Straw Waste

Bacterial synthesis of 3-hydroxybutyrate (3HB) and 3-hydroxyvalerate (3HV) copolymer [P(3HB-co-3HV)] using the hydrolysate of rice straw waste as a carbon source was affected by the composition of the hydrolysate, which depends highly on the rice straw pretreatment condition. Acid digestion with 2 % sulfuric acid generated larger production of P(3HB-co-3HV) than 6 % sulfuric acid, but 3HV concentration in the copolymer produced with 2 % acid hydrolysate was only 8.8 % compared to 18.1 % with 6 % acid hydrolysate. To obtain a higher 3HV mole fraction for enhanced flexibility of the copolymer, an additional heating was conducted with the 2 % acid hydrolysate after removal of residual rice straw. As the additional heating time increased a higher concentration of levulinic acid was generated, and consequently, the mole fraction of 3HV in P(3HB-co-3HV) increased. Among the conditions tested (i.e., 20-, 40-, 60-min), 60-min additional heating following 2 % sulfuric acid digestion achieved the highest 3HV mole fraction of 22.9 %. However, a longer heating time decreased the P(3HB-co-3HV) productivity, probably due to the increased intermediates concentrations acting as inhibitors in the hydrolysates. Therefore, the use of additional heating needs to consider both the increase in the 3HV mole fraction and the decrease in the P(3HB-co-3HV) productivity.     

The opposing effects of bacterial activity and gas production on anaerobic TCE degradation in soil columns

This laboratory study explores the effect of growth substrate concentration on the anaerobic degradation of trichloroethylene (TCE) in sand packed columns. In all columns the growth substrate rapidly degraded to gas, that formed a separate phase. Biomass accumulated in the 0–4.8 cm section of the columns in proportion to the influent growth substrate concentration and biomass concentrations in the remaining sections of all columns were similar to the column receiving the lowest substrate concentration. Increases in growth substrate concentration up to 3030 mg-COD l⁻¹ promoted TCE degradation, but a further increase to 14 300 mg-COD l⁻¹ reduced the amount of TCE completely dechlorinated but did not affect the production of chlorinated TCE intermediates. The mathematical model developed here satisfactorily described the enhancement in TCE dehalogenation for substrate concentration up to 3030 mg-COD l⁻¹; reproducing TCE dehalogenation for 14 300 mg-COD l⁻¹ required that the moisture content used in simulation be lowered to 0.1. The study shows that volatilization of TCE can be significant and volatilization losses should be taken into account when anaerobic activity in in-situ bioremediation applications is stimulated via addition of growth substrates. An implication of the modeling simulations is that maintaining a lower, but uniform, substrate concentration over the contaminated region may lead to faster contaminant degradation.     

Effect of biogeochemical interactions on bioaccessibility of arsenic in soils of a former smelter site in Republic of Korea

The total concentration-based regulations for soil remediation do not consider the possible changes in bioaccessibility of remaining arsenic (As) in soils due to biogeochemical interactions after remediation. This study used As-contaminated soil and pore water samples that were collected from the rice paddy and forest/farmland located in the vicinity of a former smelter site in Republic of Korea to elucidate the changes in As bioaccessibility due to biogeochemical interactions. Bioaccessibility and chemical forms of As in soils were determined by using an in vitro method and sequential extraction, respectively, and soil microbial community was evaluated. Bioaccessibility of As in the rice paddy soil samples was higher than that in the forest/farmland soil samples. This could be attributed to relatively higher dependence of bioaccessible As in the rice paddy soils on the soil concentration of iron (Fe), aluminum, or manganese, which could lead to greater changes in bioaccessible As via reductive dissolution. The strong linear relationship (R ² = 0.90, p value ≤0.001) between the pore water As and Fe concentrations, and the greater portion of bacterial species related to reductive dissolution of Fe oxides in the rice paddies can support the higher As bioaccessibility promoted by reductive dissolution. Therefore, it is necessary to consider the potential changes in the bioaccessible As due to biogeochemical interactions in remediation of As-contaminated soils, particularly when soils are likely to be reused under reductive dissolution-promoting conditions (e.g., flooded conditions).     

Utilization of waste bittern from saltern as a source for magnesium and an absorbent for carbon dioxide capture

During solar salt production, large quantities of bittern, a liquid by-product containing high inorganic substance concentrations, are produced. The purpose of this research was to examine the utilization of waste bittern generated from salterns as a source for Mg production and as an absorbent for carbon dioxide (CO₂) capture. The study was conducted in a sequential two-step process. At NaOH/Mg molar ratios of 2.70–2.75 and pH 9.5–10.0, > 99% Mg precipitation from the bittern was achieved. After washing with water, 100–120 g/L of precipitate containing 94% Mg(OH)₂ was recovered from the bittern. At the optimum NH₄OH concentration of 5%, 120 g of sodium bicarbonate precipitate per liter of bittern were recovered, which was equivalent to 63 g CO₂ captured per liter of bittern. These results can be used to support the use of bittern as a resource and reduce economic losses during solar salt production.