Enrichment of geogenic phosphorus in a coastal groundwater system: New insights from dissolved organic matter characterization

Identification of methane cycling pathways in Quaternary alluvial-lacustrine aquifers using multiple isotope and microbial indicators

Groundwater rich in dissolved methane is often overlooked in the global or regional carbon cycle. Considering the knowledge gap in understanding the biogeochemical behavior of methane in shallow aquifers, particularly those in humid alluvial-lacustrine plains with high organic carbon content, we investigated methane sources and cycling pathways in groundwater systems at the central Yangtze River basins. Composition of multiple stable isotopes (2H/18O in water, 13C in dissolved inorganic carbon, 13C/2H in methane, and 13C in carbon dioxide) was combined with the characteristics of microbes and dissolved organic matter (DOM) in the study. The results revealed significant concentrations of biogenic methane reaching up to 13.05 mg/L in anaerobic groundwater environments with abundant organic matter. Different pathways for methane cycling (methanogenic CO2-reduction and acetate-fermentation, and methane oxidation) were identified. CO2-reduction dominated acetate-fermentation in the two methanogenic pathways primarily associated with humic DOM, while methane oxidation was more closely associated with microbially derived DOM. The abundance of obligate CO2-reduction microorganisms (Methanobacterium and Methanoregula) was higher in samples with substantial CO2-reduction, as indicated by isotopic composition. The obligate acetate-fermentation microorganism (Methanosaeta) was more abundant in samples exhibiting evident acetate-fermentation. Additionally, a high abundance of Candidatus Methanoperedens was identified in samples with apparent methane oxidation. Comparing our findings with those in other areas, we found that various factors, such as groundwater temperature, DOM abundance and types, and hydrogeological conditions, may lead to differences in groundwater methane cycling. This study offered a new perspective and understanding of methane cycling in worldwide shallow alluvial-lacustrine aquifer systems without geothermal disturbance.

Effects of reservoir construction on optical and molecular characteristics of dissolved organic matter in a typical P-contaminated river

The source and composition characteristics of dissolved organic matter (DOM) are crucial to identify and evaluate the sources of pollution in the watershed. The construction of reservoirs changes the hydrological condition and pollutant fate of the river. However, the effects of reservoirs' construction on DOM in the watershed and the underlying mechanisms are still unclear. This study aims to examine and compare the characteristics of DOM in reservoirs and streams in the Huangbai River, a typical reservoir-affected and P-contaminated river within the Yangtze River catchment. The results showed that DOM in reservoirs was characterized by more contribution from autochthonous source, under the influence of reservoirs' construction; while, DOM in rivers was mainly originated from terrestrial input. Reservoirs had more lipid-like and protein-like compounds, while rivers contained more oxy-aromatic-like compounds. The percentage of CHOP molecules in reservoirs was significantly higher than that in rivers. The underlying mechanism is that more suitable conditions were created for plankton to grow after constructing reservoirs, which converted inorganic orthophosphate into organic phosphorus, and over time, organic phosphorus was gradually enriched in reservoirs, which exacerbated the risk of eutrophication in the reservoir water body. This study can provide theoretical support for monitoring and evaluation of water quality in reservoir-affected rivers.

Characteristics and mechanism of phosphate removal by lanthanum modified bentonite in the presence of dissolved organic matter

Lanthanum modified bentonite (LMB) is a widely used adsorbent for removing inorganic phosphorus from polluted water to prevent eutrophication. However, its efficiency can be affected by various environmental factors, including dissolved organic matter (DOM), which is still unclear. In this study, we systematically explored the influence of model DOMs, including HA, bovine serum albumin (BSA), and sodium alginate (SA), on phosphate adsorption by LMB, as well as to elucidate the underlying adsorption mechanisms. Our results showed that only HA had a significant effect on phosphate adsorption by LMB, causing inhibition. When three DOMs were mixed with phosphate in different proportions and DOM was mainly HA, the performance of phosphate adsorption on LMB became worse, while BSA can slightly offset this impact. The kinetics of HA and phosphate adsorption on LMB followed the pseudo-second-order kinetic model, and isotherms fitted the Langmuir model, with a maximum adsorption capacity of 5.7 mg g-1 for P and 12 mg g-1 for HA. However, when HA and phosphate were mixed based on their Qm, a C/P molar ratio of 5.35, LMB preferentially adsorbed phosphate. HA invasion was also disadvantageous for phosphate removal by LMB, in which P adsorption was less efficient at low-concentrations. However, during co-adsorption the adsorption capacity for HA was higher. With a secondary addition of higher levels of P, both pollutants were adsorbed more effectively. In the natural water experiment, phosphate concentration decreased with increasing shaking time, while the UV254 value showed a downward trend, indicating that LMB also adsorbed HA. Characterization results showed that La and phosphate formed LaPO4 precipitation, forming La-O-P inner-sphere complexes as the main mechanism of phosphate removal by LMB. La and HA formed La-HA complexes, with O-CO bonds from HA competing for lanthanum with phosphate. Despite HA obstructing pores from adsorbent, LMB still maintained a good binding ability with phosphate. It may form La-P-HA ternary complexes during adsorption to keep HA adsorption amount.