A Strontium and Hydro-Geochemical Perspective on Human Impacted Tributary of the Mekong River Basin: Sources Identification, Fluxes, and CO2 Consumption

As the largest and most representative tributary of the Mekong River, the Mun River Basin (MRB) provides critical understanding of regional hydro-geochemical features and rock weathering processes on a basin scale. The present study measured strontium (Sr) isotopes with hydro-geochemistry data of 56 water samples in detail in the MRB in northeast Thailand. The dissolved Sr contents and 87Sr/86Sr isotopic ratios were reported to be 8.7–344.6 μg/L (average 126.9 μg/L) and 0.7085–0.7281 (average 0.7156), respectively. The concentrations of dissolved Sr in the mainstream slightly decreased from upstream to downstream, while the variation trend of 87Sr/86Sr was on the contrary. Correlation analysis showed that Na+ strongly correlated with Cl− (0.995, p < 0.01), while Ca2+ exhibited weak relationships with SO42− (0.356, p < 0.01). Samples of the MRB exhibited lower Mg2+/Na+, Ca2+/Na+, HCO3−/Na+ and 1000Sr/Na ratios, and gathered around the end-member of evaporite dissolution, with slight shift to silicate weathering end-member, demonstrating the dominant contribution of evaporite dissolution and silicate weathering on dissolved loads. Comparing with data of major world rivers from previous research, our results remained consistency with rivers draining through similar geological conditions. The dissolved Sr flux to the adjacent Mekong River was estimated to be 20.7 tons/year. In accordance with the forward model, silicate weathering rate and CO2 consumption rate during dry season were calculated to be 0.73 tons/km2/year and 1.94 × 104 mol/km2/year, and may get underestimated due to intense water consumption by extensive agricultural activities. The superimposed effect of anthropogenic impacts on the water environment could enhance chemical weathering, and thus should be taken into account in regional ion cycles and carbon budgets. These findings highlight the coupling analysis of Sr isotopes and hydro-geochemistry in Earth surface processes and provide basic investigation for sustainable regional water treatment mechanisms in the pan basin of the Mekong River.

Nutrient Removal in Sequential Batch Polishing Ponds

One of the main problems of waste stabilization ponds (WSP) is that they cannot remove nutrients when treating wastewater. Polishing ponds (PP) can efficiently remove nitrogen and phosphorus from effluents after efficient anaerobic pretreatment. It shown that the feasibility of nutrient removal is directly related to the pH that is established in the ponds. WSP normally operate at near neutral pH, but the biological processes that develop in PP tend to cause an elevation of pH and this, in turn, triggers the mechanisms of nutrient removal in ponds. In PP oxygen production by photosynthesis predominates over the oxidation of organic material. The net oxygen production has an equivalent CO2 consumption and this induces an increase in pH. The mechanism for nitrogen removal was identified as the desorption of ammonia from the liquid phase of the ponds. It was established that in ponds with a uniform concentration profile in the liquid phase the process developed in accordance with Fick’s law. The governing mechanism of phosphorus removal was precipitation with ions present in the wastewater, presumably calcium and magnesium. Polishing ponds can be operated with two different hydrodynamic regimes: flow-through (FTPP) and sequential batch (SBPP) ponds. The SBPP have the advantage that the pH elevation is more rapid, and that the final pH is higher.

Synergetic Effect on CO2-Assisted Co-Gasification of Biomass and Plastics

CO2-assisted co-gasification of binary mixtures of pinewood pellets (PWP) and two kinds of plastics polyethylene-terephthalate (PET) and high-density polyethylene (HDPE) were examined at 800 °C using a fixed bed reactor. Evolutionary behavior and yields of CO, H2, and CmHn were investigated for both individual feedstock and binary mixtures of biomass and plastic. Synergetic effects in co-gasification of mixtures under CO2 atmosphere were analyzed and compared between experimental and calculated results. The results showed that PET and HDPE although had similar behavior in gasification, they provided many different characteristics on blending with solid biomass in CO2-assisted co-gasification. Both PWP–PET mixture and PWP–HDPE mixture showed positive effects on hydrocarbons yield and negative effects on solid yield. For PWP–PET mixture, H2 yield showed no change compared to the calculated value; however, CO yield and CO2 consumption showed negative effects due to the blocked porosity of solid biomass from the softened PET. For PWP–HDPE mixture, H2 yield showed significant enhancement compared to the calculated value, and CO yield showed slight enhancement but a slight reduction in CO2 consumption. It was also observed that the experimental CmHn yields obtained from biomass-plastics mixtures were of higher values than the calculated values. The morphologies of solid residues for PWP, PET, PWP–PET, and PWP–HDPE were analyzed and taken as a supplement to explain the synergetic effects in the co-gasification process. These results provide an insight into energy recovery and waste treatment potential for both biomass and waste plastic using thermochemical conversion.