Alteration of the molecular-size-distribution of Boom Clay dissolved organic matter induced by Na(+) and Ca(2)

Groundwater irrigation induced variations in DOM fluorescence and arsenic mobility

Dissolved organic matter (DOM) plays a predominant role in groundwater arsenic (As) mobility. However, the temporal-spatial variations in DOM fluorescent characteristics and their effects on As mobility induced by groundwater irrigation remain unclear. To address these issues, groundwater from multilevel and irrigation wells in Zones I and II (with low- and high-As groundwater irrigation, respectively) from the Hetao Basin, China, were monitored in both non-irrigation (NIG) and irrigation (IG) seasons. Upon irrigation, the irrigation return increased the relative abundance of protein- and humic-like DOM in shallow groundwater from Zone I with Ca-type groundwater and Zone II with Na-type groundwater irrigation, respectively. The introduced dissolved oxygen by irrigation return decreased As concentrations by 22 % and 6 % on average in shallow groundwater from Zones I and II, respectively. However, the pumping-induced lateral recharge of lower- and higher-As groundwater led to an average 17 % decrease and 38 % increase in As concentrations in deeper groundwater from the two zones, respectively. The increased degradation of protein-like DOM may also contribute to the elevated As concentrations in deep groundwater from Zone II. The study provides insights into the dependence of irrigation-induced variations in DOM fluorescence and As concentrations on geochemicals of irrigation groundwater and aquifer hydrogeological conditions.

Ca/Na concentration-constrained variations of dissolved organic matter leaching from groundwater-irrigation area soil in North China Plain

This study investigates the quantity and quality variations of dissolved organic matter (DOM) leaching from the soil in groundwater irrigation area of the North China Plain, constrained by the concentration of Ca/Na. Soil samples with dominant humic-like (HLC) and protein-like (PLC) components were paired with parallel concentration gradients of Ca/Na extractants for equilibrium experiments. Fluorescence-PARAFAC, UV-visible spectroscopy, and multiple statistical analyses were combined for data analysis and interpretation. The results reveal that the primary DOM components remained dominant for specific soil sample, with a higher relative abundance of PLC (HLC) in Ca (Na) extract. HLC preferentially binds to soil phase in all extractions, while PLC is readily released into the solution. However, Ca inhibits HLC desorption and promotes PLC release more significantly than Na, as indicated by stronger ion/proton reaction (IPR) and electrostatic effect (ESE). The strongest IPR and ESE are seen in the HLC-dominated DOM extracted with Ca, suggesting a condition where Ca bridges to HLC and forms total dissolved organic carbon (DOC) that decreases. In contrast, Na extraction exhibits only a weaker ESE that is offset by soil-contained HLC and exchangeable Ca, resulting in subtle DOC decrease. The trends in leaching of HLC and PLC are self-dependent, and the level of variation in either component correlates with the increasing concentration of specific cations present. These findings underscore the crucial role of soil organic matter (SOM) composition and its interaction with leaching cations in soil management in large-scale groundwater irrigation areas, where SOM quality and groundwater chemistry vary.

In-depth understanding of local soil chemistry reveals that addition of Ca may counteract the mobilisation of 226Ra and other pollutants before wetland creation on the Grote Nete river banks

The "Sigma plan" https://www.sigmaplan.be/en/ aims to create in Belgium inundation zones along the Grote Nete river to prevent Antwerp from flooding in extreme weather conditions. The riverbanks of the Grote Nete are at some hotspots historically contaminated by the phosphate industry resulting in Naturally Occurring Radionuclides (NOR) legacy. ²²⁶Ra is from a radiation protection point of view one of the most important radionuclides present at the hot spot under study, with a local soil activity concentration higher than 3000 Bq/kg ²²⁶Ra. In this paper, we identify the most relevant mechanisms governing the mobility of ²²⁶Ra. We selected for this study the role of CaSO₄.2H₂O, clay minerals and humic acids as the main contributors determining the speciation of Ra, due to their presence at the hot spot, their cation exchange capacity and their functional group density, respectively. Various novel analytical chemistry approaches were developed to study the prevailing reaction mechanisms that impact the solid-liquid distribution of ²²⁶Ra. We show that ²²⁶Ra coprecipitates in a (Ca,Ra)SO₄ solid solution due to the high Ca²⁺ and SO₄^2- concentrations in the local hot spot. If CaSO₄.2H₂O is not saturated in the soil solution, ²²⁶Ra adsorption to clay minerals counteracts the tendency of ²²⁶Ra partitioning to the liquid phase by interactions with humic and fulvic acids. Interactions between different soil compounds may further alter the partitioning of Ra. As, Cd, Pb and Zn in the hot spot are significantly above background values in Flemish sediments. Pb may be coprecipitated as sulphate salts, whereas Cd and Zn are most probably partially present as arsenate salts. The excess of Zn may interact with humic acids. The observed reaction mechanisms suggest that Ca²⁺ might play a key role in the immobilisation of Ra. The role of Ca²⁺ as immobilisation agent of the other contaminants is discussed.

Rice seed (IR64) priming with potassium humate for improvement of seed germination, seedling growth and antioxidant defense system under arsenic stress

The intolerable levels of arsenic (As) in groundwater and its application in rice cultivation are continuously affecting the rice production in Ganga-Meghna-Brahmaputra (GMB) plain. The reduced germination and plant growth rates under excessive As stress ultimately lead to lower yield. To mitigate this concerning issue, the present study was carried out to evaluate the potential of K-humate priming on seed germination and plant growth under As stress. Seeds were primed with 100 mg/l K-humate for 12 h prior to germination. The germination percentages in unprimed seeds were 65 ± 5.0% and 58.3 ± 7.6% under stress level of 50 μM AsV and 50 μM AsIII, respectively. However, germination percentage in K-humate primed seeds were 75 ± 5.0% and 68.3 ± 2.9% under AsV and AsIII stress, respectively. The vigour index I (VG I) and vigour index II (VG II) recorded on 12 DAS (days after seeding) were also increased by 1.47 and 1.51 fold, respectively with K-humate supplementation under As stress. Detrimental effects of AsIII on seed germination, seedling growth and other physiological parameters were more suppressive than AsV. Application of K-humate not only improved seed germination, seedling growth and nutrient uptake but also decreased the oxidative stress markers and antioxidant activities by minimizing As uptake and translocation in the seedlings.

Toward a microscopic model of light absorbing dissolved organic compounds in aqueous environments: theoretical and experimental study

Water systems often contain complex macromolecular systems that absorb light. In marine environments, these light absorbing components are often at the air-water interface and can participate in the chemistry of the atmosphere in ways that are poorly understood. Understanding the photochemistry and photophysics of these systems represents a major challenge since their composition and structures are not unique. In this study, we present a successful microscopic model of this light absorbing macromolecular species termed "marine derived chromophoric dissolved organic matter" or "m-CDOM" in water. The approach taken involves molecular dynamics simulations in the ground state using on the fly Density Functional Tight-Binding (DFTB) electronic structure theory; Time Dependent DFTB (TD-DFTB) calculations of excited states, and experimental measurements of the optical absorption spectra in aqueous solution. The theoretical hydrated model shows key features seen in the experimental data for a collected m-CDOM sample. As will be discussed, insights from the model are: (i) the low-energy A-band (at 410 nm) is due to the carbon chains combined with the diol- and the oxy-groups present in the structure; (ii) the weak B-band (at 320-360 nm) appears due to the contribution of the ionized speciated form of m-CDOM; and (iii) the higher-energy C-band (at 280 nm) is due to the two fused ring system. Thus, this is a two-speciated formed model. Although a relatively simple system, these calculations represent an important step in understanding light absorbing compounds found in nature and the search for other microscopic models of related materials remains of major interest.

Surface properties and suspension stability of low-temperature pyrolyzed biochar nanoparticles: Effects of solution chemistry and feedstock sources

Intensive application of biochar requires better understanding of their environmental behaviors such as stability, fate, and mobility. The release of bulk biochar into biochar nanoparticles (NPs) may bring risks because of their potential flowing into downstream water bodies with nutrients/containments attached. Low-temperature pyrolyzed biochars, namely fruit tree branch biochar of 350/450/550 °C (FB350, FB450 and FB550), corn straw biochar of 350 °C (CB350) and peanut straw biochar of 350 °C (PB350), were produced, and their NPs were extracted. The yield, elemental composition, mineral composition, surface functional groups and zeta potential of biochar NPs were characterized. Subsequently their suspension stability was evaluated in NaCl and CaCl₂ solutions by dynamic light scattering technique. The Hamaker constants and particle interaction energy of the biochar NPs were calculated by adopting Derjaguin-Landau-Verwey-Overbeek theory. For biochar NPs of same feedstock, the stability of FB350/450/550-NPs could be predicted well by their zeta potential values. The types of their surface functional groups were the same while their adsorption intensity differed. The scenarios for biochar NPs of different feedstock sources were different, that is, inconsistent variation was observed between their zeta potential and suspension stability, which were rooted in the variable type and quantity of surface functional groups. In conclusion, feedstock was the most significant factor that influenced the suspension stability of biochar NPs, followed by the pyrolysis temperature and solution chemistry, which were highly dependent on surface potential. The findings provide references for the environmental risk evaluation of biochar NPs and reasonable application of biochar in field.

Structural characteristics, analytical techniques and interactions with organic contaminants of dissolved organic matter derived from crop straw: a critical review

Dissolved organic matter (DOM) represents one of the most mobile and reactive organic compounds in an ecosystem and plays an important role in the fate and transport of soil organic pollutants, nutrient cycling and more importantly global climate change. Advances in environment geochemistry in the past two decades have improved our knowledge about the genesis, composition, and structure of DOM, and its effect on the environment. Application of analytical technology, for example UV-visible spectroscopy (UV-Vis), Fourier transform infrared spectroscopy (FTIR), Nuclear magnetic resonance (NMR) spectroscopy, and three-dimensional fluorescence spectroscopy (3D-EEM) have resulted in these advances. At present, crop straw, as a part of energy development strategy, is mainly used for soil amendment, fodder, fertilizer and industrial materials. Moreover, the fermentation and decomposition of straw should be also promoted for ecological agriculture. However, few studies have focused on the structural properties of DOM derived from crop straw in farmland soil. In this article, DOM derived from crop straw, which is abbreviated to "CDOM", presents active physicochemical properties that can affect the migration and bioavailability of organic contaminants (OCs) in terrestrial ecosystems. The objectives of this review paper are: (i) to discuss the structural characteristics, analytical techniques and interactions between CDOM and OCs in farmland soil; (ii) to present a critical analysis of the impact of CDOM on the physicochemical transformation and transport of OCs in farmland soils; (iii) to provide the perspectives in future research. Therefore, the findings obtained from this study can be utilized to evaluate the relations of interactions between CDOM and OCs in agricultural soils, in order to support some suggestions for future development in agricultural waste recycling, buffering of organic pollution, and the effect on the global carbon cycle.

Size and Charge Evaluation of Standard Humic and Fulvic Acids as Crucial Factors to Determine Their Environmental Behavior and Impact

In this work, the size and charge of humic and fulvic standards purchased from the International Humic Substances Society are presented and discussed. The secondary structure of humic substances in water environment as well as the size and shape of the dissolved humic species and their changes are ill-defined, very dynamic and can be strongly affected by environmental conditions as the concentration, pH, and ionic strength. They have a strong propensity to aggregate which control their interactions with other components, mobility, and functioning in the environment. Particle size distributions were determined by means of dynamic light scattering, zeta potential by Doppler electrophoresis. The intensity, volume, and number particle size distribution were obtained. Two or three different size fractions were detected in the studied systems. Large macroparticles (>1 μm) were observed in the majority of them, mainly in the case of more concentrated solutions. Medium fractions of fulvic submicroparticles had higher average diameters (500–1,200 nm) than those in humic acids (300–600 nm). Small nanoparticles (<100 nm) were detected mainly in alkaline solutions. Fulvic acids with more functional groups (active sites) can form more easily bigger particles mainly in medium concentration region. Alkaline conditions supported the expansion of humic and fulvic coils and liberation of small particles from them. The colloidal stability, indicated by more negative zeta potentials, was higher for humic acids. In the case of fulvic acids, the colloidal stability increased with increasing pH as a result of the dissociation of their functional groups. The increase of particle size corresponded usually with higher stability.

Transport of dissolved organic matter in Boom Clay: Size effects

A coupled experimental-modelling approach was developed to evaluate the effects of molecular weight (MW) of dissolved organic matter (DOM) on its transport through intact Boom Clay (BC) samples. Natural DOM was sampled in-situ in the BC layer. Transport was investigated with percolation experiments on 1.5cm BC samples by measuring the outflow MW distribution (MWD) by size exclusion chromatography (SEC). A one-dimensional reactive transport model was developed to account for retardation, diffusion and entrapment (attachment and/or straining) of DOM. These parameters were determined along the MWD by implementing a discretisation of DOM into several MW points and modelling the breakthrough of each point. The pore throat diameter of BC was determined as 6.6-7.6nm. Below this critical size, transport of DOM is MW dependent and two major types of transport were identified. Below MW of 2kDa, DOM was neither strongly trapped nor strongly retarded. This fraction had an averaged capacity factor of 1.19±0.24 and an apparent dispersion coefficient ranging from 7.5×10^-11 to 1.7× 10^-11m²/s with increasing MW. DOM with MW>2kDa was affected by both retardation and straining that increased significantly with increasing MW while apparent dispersion coefficients decreased. Values ranging from 1.36 to 19.6 were determined for the capacity factor and 3.2×10^-11 to 1.0×10^-11m²/s for the apparent dispersion coefficient for species with 2.2kDa<MW<9.3kDa. Straining resulted in an immobilisation of in average 49±6% of the injected 9.3kDa species. Our findings show that an accurate description of DOM transport requires the consideration of the size effects.