Coprecipitation Of Organic Matter Research Articles

Coprecipitation of Organic Matter, Phosphate With Iron: Implications for Internal Loadings of Phosphorus in Algae-Dominated and Macrophyte-Dominated Lakes

Coprecipitation with iron (Fe) plays an essential role in the biogeochemical cycles of organic carbon (OC) and phosphorus (P) in lakes. The sources and composition of organic matter (OM) mediate its association with iron, which could thus influence the immobilization of phosphorus. In this study, water-soluble organic matter from the sediments of two typical states of shallow lakes, macrophyte-dominated zones (M-WSOM) and algae-dominated zones (A-WSOM), was extracted, and the ternary coprecipitation of WSOM, phosphate with Fe(III), was investigated. The ternary coprecipitation process was enhanced with increasing Fe(III) or decreasing pH value. It was found that pH of 6.5 was more favorable for coprecipitation than a pH of 7.5 or 8.5. At pH 6.5, the complexation between WSOM and Fe(III) occurred at the low Fe(III) inputs, while the coprecipitation of phosphate, WSOM with Fe(III) took place when Fe(III) inputs reached 40 μM. The presence of A-WSOM showed stronger inhibition on the coprecipitation of phosphate than that by M-WSOM. The formed ternary coprecipitates with A-WSOM had lower C/Fe ratios (0.13–2.78) than those with M-WSOM (1.28–4.05), which was because A-WSOM had lower aromaticity than M-WSOM. In addition, more functional groups in A-WSOM could complex with Fe(III), resulting in less immobilization of OC and P during the coprecipitation of A-WSOM, phosphate, and Fe(III). Our results demonstrated that in algae-dominated zones, more phosphorus remained soluble during the ternary coprecipitation, which could perform positive feedback on the growth of phytoplankton and provide a novel explanation for the difficulty in restoring eutrophic lakes.

Characterization of goethite-fulvic acid composites and their impact on the immobility of Pb/Cd in soil

The coprecipitation of organic matter (OM) and minerals is a relatively common phenomenon in soil, and it has a significant influence on the surface properties and reactivity of minerals. In turn, the fate of pollutants in soil is greatly affected by the organic-mineral composites. In this study, goethite-fulvic acid (Ge-FA) composites with varying FA mass ratios in the range of 0–15% were synthesized by coprecipitation. The sample properties were studied using XRD, FTIR, SEM-EDS and N2 gas adsorption techniques. The influence of Ge-FA on the mobility of Pb/Cd in soil was investigated. The crystal forms of Ge-FA changed from goethite (FA≤4%) to hematite (FA≥5%), and the FA affected the FeO bond vibrations. These results demonstrated that FA was successfully introduced into the iron oxide. Ge-FA changed from a filamental morphology to an aggregate as the FA ratio increased. The coprecipitation resulted in blockages of iron oxides, thereby decreasing the specific surface area and pore volume. The adsorption amount of Pb(II) on Ge-FA increased as the FA ratio increased, but no significant change was observed for Cd(II). With the application of Ge-FA, the exchangeable concentrations of Pb and Cd in contaminated soil decreased by 42.4%–93.6% and 15.8%–43.7%, respectively. The exchangeable and carbonate bound fractions of Pb and Cd decreased and were transformed into the FeMn bound and residual fractions.

Binding of Cd by ferrihydrite organo-mineral composites: Implications for Cd mobility and fate in natural and contaminated environments

Adsorption and coprecipitation of organic matter with iron (hydr)oxides can alter iron (hydr)oxide surface properties and their reactivity towards nutrient elements and heavy metals. Organo−mineral composites were synthesized using humic acid (HA) and iron oxide, during coprecipitation with ferrihydrite (Fh) and adsorption to pre-formed Fh with two C loadings. The Fh−HA coprecipitated composites have a higher C content and smaller surface area compared to the equivalent adsorbed composites. NanoSIMS shows there is a high degree of spatial correlation between Fe and C for both composites, but C distribution is more uniform in the coprecipitated composites. The C 1s NEXAFS reveals a similar C composition between the Fh−HA coprecipitated and adsorbed composites. However composites at high carbon loading are more enriched in aromatic C, likely due to preferential binding of carboxyl functional groups on aromatic rings in the HA. The amount of Cd sorbed is independent of the composite type, either coprecipitated or adsorbed, but is a function of the C loading. Composites with low C loading show Cd sorption that is almost identical to pure Fh, while composites with high C loading show Cd sorption that is intermediate between pure Fh and pure HA, with sorption significantly enhanced over pure Fh at pH < 6.5. A bidentate edge-sharing binding was identified for Cd on pure Fh and Cd-carboxyl binding on pure HA. These findings have significant implications not only for the sequestration of Cd in contaminated environments but also the coupled biogeochemical cycling of Cd, Fe and C in the critical zone.

Dissimilatory Reduction and Transformation of Ferrihydrite-Humic Acid Coprecipitates

Organic matter (OM) is present in most terrestrial environments and is often found coprecipitated with ferrihydrite (Fh). Sorption or coprecipitation of OM with Fe oxides has been proposed to be an important mechanism for long-term C preservation. However, little is known about the impact of coprecipitated OM on reductive dissolution and transformation of Fe(III) (oxyhydr)oxides. Thus, we study the effect of humic acid (HA) coprecipitation on Fh reduction and secondary mineral formation by the dissimilatory Fe(III)-reducing bacterium Shewanella putrefaciens strain CN32. Despite similar crystal structure for all coprecipitates investigated, resembling 2-line Fh, the presence of coprecipitated HA resulted in lower specific surface areas. In terms of reactivity, coprecipitated HA resulted in slower Fh bioreduction rates at low C/Fe ratios (i.e., C/Fe ≤ 0.8), while high C/Fe ratios (i.e., C/Fe ≥ 1.8) enhanced the extent of bioreduction compared to pure Fh. The coprecipitated HA also altered the secondary Fe mineralization pathway by inhibiting goethite formation, reducing the amount of magnetite formation, and increasing the formation of a green rust-like phase. This study indicates that coprecipitated OM may influence the rates, pathway, and mineralogy of biogeochemical Fe cycling and anaerobic Fe respiration within soils.

Natural organic matter (NOM) in a limed lake and its tributaries

The chemistry of a limed lake and its main tributaries were studied for 3 years (1992–94) with an emphasis on natural organic matter (NOM). Increased transparency and decreased water colour indicated a general reduction of NOM in the lake. Increased A 254 nm /A 410 nm ratios in the epilimnion during summer and early autumn suggested degradation of higher molecular size organic matter into low molecular size NOM. Increase in ammonium and dissolved inorganic carbon concentrations in the lake was possibly due to the NOM degradation. Using budget calculations and the literature values, photodegradation and microbial activity were estimated to be the main mechanisms of the NOM removal. These mechanisms accounted for about 30–35% and 60–65% of the total loss of organic matter, respectively, in the summer and early autumn period. Low sedimentation rates indicate that co-precipitation of organic matter with calcium, aluminium and/or iron was of minor importance in these seasons.

Interactions of ferric and ferrous irons and organic matter in water environment

Coprecipitation of organic matter with ferric and ferrous hydroxide, and iron holding in solution containing organic matter were experimentally studied. The dissolved organic matter from bacterial degradation of green algae was used. The results of the experiments showed that ferrous iron, as well as ferric iron, coprecipitate organic matter under reduced conditions, and that ferric and ferrous irons selectively coprecipitate proteinous materials. On the contrary, pigment and lipid materials are rarely coprecipitated, and hold ferric iron in dissolved form probably by forming water soluble iron-organic complexes. These results show the significance of the interaction of organic matter and iron in the cycle of them in the hydrosphere.