Colloidal Research Articles

Leaching and vertical distribution of Fe and Zn citrate nanoparticles in Indian red soil

This research aims to address common constraints in the effective utilization of plant nutrients in soil, such as fixation, mobility, leaching, and reactions with soil colloids. To mitigate these issues, Fe and Zn citrate nanoparticles were synthesized and applied as nanochelators in a reconstructed soil profile column. We evaluated the mobility, release, and leaching behaviors of these nanoparticles. Results revealed that, no leaching of Fe and Zn citrate nanoparticles occurred even after a 90-day incubation. The release profiles exhibited a peak at 60 and 90 days for Fe and Zn respectively. In the mobility studies, Fe and Zn availability was highest in the 0–15 cm soil depth. Fe citrate and Zn citrate nanoparticles demonstrated the highest availability at 264.7 mg/kg and 86.26 mg/kg of soil, respectively as compared with commercial samples. The superior performance of Fe and Zn citrate nanoparticles was observed in terms of reduced leaching and improved accessibility, indicating their potential as efficient and environmentally-friendly plant nutrient sources. The study concludes that Fe and Zn citrate nanoparticles are stable nutrient sources that can enhance plant use efficiency with minimal environmental impact.

Combined effects of soil colloid and soil extracellular enzymes on nitrogen loss from sloping farmland

The extracellular enzyme plays a crucial role in nitrogen (N) conversion. Soil colloid serves as an important transporter of N transport in hydrological processes. This study investigated soil colloid-mediated N loss co-transporting with soil extracellular enzymes. Five simulated rainfall experiments were conducted under four tillage treatments in a purple sloping farmland in Sichuan, China. The N concentrations, soil mineral colloids, and four carbon (C), N and phosphorus (P) acquisition extracellular enzymes (βG, AP, NAG, and LAP) in surface runoff and interflow were measured. The results showed that cross-slope tillage with straw returning practices significantly reduced the concentrations of TN, PN, NH4+, and DON in surface runoff. The activities of N and P acquisition enzymes in interflow were higher than in surface runoff, while C acquisition enzymes showed the opposite trend. The BG and AP enzymes dominated in surface runoff, while AP and NAG dominated in interflow. The concentrations of fine soil mineral colloids (SMC, φ<1 μm) and coarse mineral colloids (CMC, φ＞1 μm) in interflow were higher than that in surface runoff. The extracellular enzymes were found to co-transport with soil colloid migration during the hydrologic process. The involvement of colloid in extracellular enzyme migration in surface runoff was primarily due to SMC, while in interflow, it was the joint action of SMC and CMC. Surface runoff is always in N and P limits, while interflow is only in the P limits. With a SEM combined model quantitatively analysis, we found the synergistic transport of soil colloid and extracellular enzymes significantly impacted TN loss, explaining 95 % and 55 % of the differences between surface runoff and interflow N loss pathways. This emphasizes the importance of understanding the co-transport mechanism between soil colloid and extracellular enzymes in N loss processes.

Cotransport of Thallium(I) and kaolinite colloids in quartz sand media containing sodium humate: Ionic strength, pH and kaolinite colloid concentration

In real soil environments, humus, colloids and other components significantly affect pollutant migration behavior. Investigating Tl(I) and kaolinite colloids’ cotransport in quartz sand media containing sodium humate (HA-Na) is vital for comprehending Tl(I) migration underground. This study examined the migration of Tl(I) and kaolinite colloids across varying pH levels (5, 7), ionic strengths (ISs) (1, 5, 50 mmol/L), and kaolinite colloid concentrations. Results indicate that lower IS and pH promote Tl(I) migration when transported alone. In cotransport system, kaolinite promotes Tl(I) migration under acidic conditions but inhibits it under neutral conditions, except at high kaolinite concentrations, where the effect shifts from inhibition to promotion. This is primarily due to changes in the zeta potential of quartz sand, HA-Na, and kaolinite, as well as Tl(I) adsorption after HA-Na and kaolinite occupy binding sites. Competitive adsorption between cations and Tl(I) also plays a significant role. Conversely, in individual system, higher IS and pH inhibit kaolinite migration, while increased kaolinite concentration promotes it. In cotransport system, Tl(I) promotes kaolinite migration under acidic conditions but inhibits it under neutral conditions, except at low kaolinite concentrations. This relates to changes in the zeta potential between kaolinite and the medium, as well as the retention of HA-Na in the column and its adsorption onto kaolinite. Competitive adsorption and binding site saturation also have an impact. This study enhances understanding of Tl(I) migration by revealing the dual effect of kaolinite colloids under different environmental conditions, contributing to better knowledge of Tl(I) fate and transport in natural environments.

Control mechanism of short-term fertilization with cattle manure on the release characteristics of soil colloids in farmland: grain size and physicochemical properties

BackgroundUnderstanding the release characteristics of soil colloids is a prerequisite for studying the co-transport of colloids and pollutants in subsurface environment. As a crucial agricultural management measure, fertilization not only alters the material composition of farmland soil, but also significantly regulates the properties and release patterns of soil colloids. This study systematically investigated the regulatory mechanism of short-term cattle manure fertilization on the macroscopic release and microscopic properties of soil colloids with different particle sizes, providing a theoretical foundation for subsequent research on the fate and transport of agricultural non-point source pollutants.ResultsThe colloids in natural agricultural soil primarily consist of inorganic components. Graded extraction of the colloids has revealed that the combined proportion of colloids with particle sizes of 1–2 μm and 0.45–1 μm accounts for approximately 80.5%. Applying cattle manure inhibits the release of soil colloids, and the content of large particle size (1–2 μm) components increases. The content of organic colloids is increased due to the high total organic carbon (TOC) in cattle manure, particularly those with a particle size less than 1 μm. The characterization of organic colloid components revealed a significant increase in aromatic carbon and oxygen-containing functional groups, while the aliphatic content decreased. The response sequence regarding changes in functional groups within organic colloids induced by fertilization was as follows: –CH3, –CH2 > C–O > –OH > C=C. Fertilization promotes the release of 1:1-type inorganic mineral colloids, increasing the content of poorly crystalline minerals. The retention of aromatic carbon and oxygen-containing functional groups by poorly crystalline mineral colloids served as the primary mechanism leading to their increased content levels. Changes in environmental factors significantly impacted the release and properties of soil colloids. Conditions such as low cationic valence, high ionic strength, and high pH promoted the release of soil colloids.ConclusionsThe short-term fertilization resulted in a reduction in the release of soil colloids and brought about significant alterations in their particle size composition and properties. The findings of this study provide valuable insights into understanding the impact of fertilization-induced colloid release on the environmental behavior of agricultural non-point source pollutants.

The heterogeneous distribution of heavy metal(loid)s at a smelting site and its potential implication on groundwater

The downward migration of soil heavy metal(loid)s (HMs) at smelting sites poses a significant risk to groundwater. Therefore, it is requisite for pollution control to determine the pollution characteristics of soil HMs and their migration risks to groundwater. 198 soil samples collected from a Pb-Zn smelting site were classified into 6 clusters by self-organizing map (SOM) and K-means clustering. Cd, Zn, As, and Pb were identified as the characteristic contaminants of the site. The driving factors for the heterogeneous distribution of HMs have been validated through the implementation of K-means clustering and multiple-hits calculation. Using ultrafiltration extraction and microscopic analysis, the soil colloids were identified as crucial carriers facilitating the migration of HMs. Specifically, the colloidal fractions of Cd, Zn, and As, Pb in deep soil (3–4 m) accounted for 91 %, 78 %, 88 %, and 82 %, respectively, consistently surpassing those found in topsoil (0–0.5 m). It was primarily attributed to the strong affinity of HMs toward soil colloids (franklinite, PbS, and kaolinite) and dissolved organic matter (humic acids and protein). The research findings highlight the potential risk of colloidal HMs to groundwater contamination, providing valuable insights for the development of targeted management and remediation strategies.

Heavy Metal Contamination in Soil and Health Risk Assessment through Onion Consumption in Mojo and Koka, Ethiopia

The objective of this study was to determine the concentration of heavy metals in the soil and horticultural crops grown under irrigation in the Mojo and Koka sites in Oromia, Ethiopia, and to evaluate the potential health risks that these metals may cause to consumers. For this investigation, a total of 50 onion (Allium porrum L.) and 50 soil samples were gathered. The content of heavy metals was measured using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) after the samples were digested using microwave-assisted digestion. The mean concentrations of examined trace elements in onion (mg/kg) were found in the range of 0.80 to 0.95, for As 1.77 to 0.91 for Sn, 2.09 to 1.49 for Pb, 12.29 to 7.31 for B, 13.85 to 12.33 for Zn, 0.34 to 0.25 for Cd, 1.83 to 1.78 for Hg, 11.29 to 7.54 for Cu, 0.83 to 0.78 for Ni, 0.34 to 0.31 for Co, 78.39 to 42.19 for Fe, 17.03 to 11.56 for Mn, and 1.03to 1.02 for Cr. Given that the average levels of Pb, Hg, Cd, and As in onions at both irrigation sites surpassed the upper limit of allowable levels established by the joint FAO/WHO commission. These metals’ concentrations in soil are found to be within acceptable bounds. As compared to other literatures, their concentration exceeded in both irrigation sites. According to the metal pollution load index, the overall pollution load of trace metals, including As, Pb, Cd, and Hg, was found to be moderately higher in both irrigation locations for the majority of the onions under study. The results, which were computed using an adult consumer’s food intake, indicated that the Pb and Hg dietary exposure levels were much higher than the daily intake amounts that were considered tentatively tolerated. The soil-to-onion transfer factor showed that whereas Fe has a strong sorption to the soil colloids, metal Cu is comparatively poorly maintained in the soil. The degree of metal enrichment in soils and sediments is higher than in other literature, according to the soil pollution indices. Thus, swift action is needed to stop the issue and guarantee public safety along the food system’s supply chain.

Characterizing the Stoichiometry of Individual Metal Sulfide and Phosphate Colloids in Soils, Sediments, and Industrial Processes by Inductively Coupled Plasma Time-of-Flight Mass Spectrometry.

Size and purity of metal phosphate and metal sulfide colloids can control the solubility, persistence, and bioavailability of metals in environmental systems. Despite their importance, methods for detecting and characterizing the diversity in the elemental composition of these colloids in complex matrices are missing. Here, we develop a single-particle inductively coupled plasma time-of-flight mass spectrometry (sp-icpTOF-MS) approach to characterize the elemental compositions of individual metal phosphate and sulfide colloids extracted from complex matrices. The stoichiometry was accurately determined for particles of known composition with an equivalent spherical diameter of ≥∼200 nm. Assisted by machine learning (ML), the new method could distinguish particles of the copper sulfides covellite (CuS), chalcocite (Cu2S), and chalcopyrite particles (CuFeS2) with 75% (for Cu2S) to 99% (for CuFeS2) accuracy. Application of the sp-icpTOF-MS method to particles recovered from natural samples revealed that iron sulfide (FeS) particles in lake sediment contained ∼4% copper and zinc impurities, whereas pure pyrite (FeS2) was identified in hydraulic fracturing wastewater and confirmed by selected area electron diffraction. Colloidal mercury in an offshore marine sediment was present as pure mercury sulfide (HgS), whereas geogenic HgS recovered from an industrial process contained ∼0.08 wt % silver per Hg, enabling source apportionment of these colloids using ML. X-ray absorption spectroscopy confirmed that Hg was predominantly present as metacinnabar (β-HgS) in the industrial process sample. The determination of impurities in individual colloids, such as zinc and copper in FeS, and silver in HgS may enable improved assessment of their origin, reactivity, and bioavailability potential.

The complex effect of dissolved organic carbon on desorption of per- and poly-fluoroalkyl substances from soil under alkaline conditions

Per- and poly-fluoroalkyl substances (PFASs) are contaminants of emerging concern, yet the understanding of factors that control their leaching and release from contaminated soils remains limited. This study aimed to investigate the impact of dissolved organic carbon (DOC) on the release of PFASs—specifically, perfluorohexane sulfonate (PFHxS), perfluorooctane sulfonate (PFOS), and perfluorooctanoic acid (PFOA)from soils contaminated by aqueous film forming foam (AFFF). Batch aqueous leaching experiments were conducted on AFFF-contaminated soils under alkaline solution conditions (pH 9.5, 10.5, and 12) as it enhances leaching of both PFAS and DOC. Leaching of PFOS was significantly increased under alkaline conditions. Although the leaching of PFAS generally increased with pH, PFOS appeared to be more retained under the very alkaline pH conditions used in this study. At the same solution pH, leaching of PFOS and DOC was less in Ca(OH)2 than in NaOH. The retention of PFOS under these conditions may be attributable to the shielding of the negative charge of the soil components and colloids (e.g., DOC and clay minerals) in the leachates and/or the screening of negative charges on head groups of PFOS due to the high concentration of divalent cations. Solution chemistry affected desorption of PFOS more than PFHxS and PFOA. The study highlights that the influence of DOC on PFAS leaching and transport can be very complex, and depends on leachate chemistry (e.g., pH and cation type), PFAS chemistry, the magnitude of PFAS contamination and factors that influence the solid:liquid partitioning of organic carbon in soil.

Mineralogical Characteristics of Soils in Southern Taraba State Northeast, Nigeria

Mineralogy of fadama soils of southern Taraba state was carried alongside its influence on the soil’s physical and chemical properties. The three local government areas selected for this study were Ibi, Wukari and Donga. The mineralogical compositions of the soils were evaluated using the clay fraction. X- ray diffraction (XRD) analysis showed ibi had quartz (21%), microcline (47%) and kaolinite (25%), Wukari Tsukundi had quartz (17%), microcline (20%) and kaolinite (32%), phlogopite (15.4%) and gibbsite (23%) while Donga had mineral composition of quartz (20%), microcline (33%) and kaolinite (47%) and therefore the clay minerals are interstratified (mixed mineralogy). The soil textures in these mapping units (MUs) are sandy clay loam (SCL), loam sand (LS) and sandy loam (SL) in the Ap horizons of pedons I, 2 and 3 respectively. The subsurface horizons are Silt Loam, SCL and LS. The silt clay ratio (SCR) indicated that the soils are relatively young soils with reserved weatherable minerals, mean SCR were 1.40, 1.50 and 4.00 in pedons 1, 2 and 3 respectively. The acidity decreased as the profile depth increased and it was moderately acidic soil with pH(KCl) ranged from 5.92- 4.75 for pedons. The delta pH (\(\Delta\)pH) of the soils showed all negative values; this indicated that the soil colloids contained appreciable silicate clay minerals with relatively constant surface charge while C/N ratios indicated advanced stage of organic matter decomposition. Percentage Al saturation to the TEA stood at 15%, 19% and 9% for pedons I, 2 and 3 respectively, The exchangeable Ca constitutes above 70 % of the total exchangeable bases (TEB). The percentage base saturation (%BS), was largely dominated by exchangeable cations in moderately to very high rating and their mean values are 80%, 77% and 74% for pedons I, 2 and 3 respectively. The arrays of minerals are at different stages of weathering- quartz and phlogopite at moderate stage of weathering, microcline at minimal stage of weathering while kaolinite and gibbsite are at intense weathering stages. These minimally and moderately weathered soils are generally considered to be of good fertility status. The soils were classified as Alfisols/Entisols mix.

Optical and molecular insights into dissolved organic matter release in soils induced by downward migration of biochar colloids

The frequent wildfires and extensive biochar application accumulate biochar colloids in soils. The transport of biochar colloids may influence the release of soil dissolved organic matter (DOM) due to their high mobility and active surface, posing a detrimental impact on carbon cycle, pollutants behaviors, and water quality. Here, we examined the effect of pristine and aged biochar colloids on soil DOM release in three types of soils (black soil, fluvisol, and paddy soil) through adsorption and column experiments combined with fluorescence excitation/emission matrix spectroscopy (EEM) and fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). Adsorption experiment showed that biochar exhibited little effect on soil DOM release, while transport experiment indicated that the downward movement of both pristine and aged biochar colloids significantly enhanced the release of soil DOM, especially black soil and fluvisol. EEM spectra indicated that a humic acid-like substance with large molecular weights was the primary DOM that enhanced release from black soil (10.9–12.0% enhancement) and fluvisol (6–8% enhancement) during biochar colloid transport. Condensed aromatic-like (22.8% and 19.4%) and tannin-like (18.4% and 18.9%) compounds are the primary molecules correlated to the enhanced release of humic acid-like substances. Although chemical aging resulted in a more negative surface of aged biochar colloids and slightly enhanced biochar colloid transport (up to 7.0%), it showed no distinct effect on the release of soil DOM. This study indicates that biochar colloids may cause an increase in DOM release when irrigation and rainfall occur, and more attention should be paid to the environmental impacts of biochar colloids from intensive biochar application and wildfires.Graphical

Effects of soil colloids on adsorption and migration of benzo(a)pyrene on contaminated sites under runoff infiltration processes

In the environment, soil colloids are widespread and possess a significant adsorption capacity. This makes them capable of transporting different pollutants, presenting a potential risk to human and ecological well-being. This study aimed to examine the adsorption and co-migration characteristics of benzo(a)pyrene (BaP) and soil colloids in areas contaminated with organic substances, utilizing both static and dynamic batch experiments. In the static adsorption experiments, it was observed that the adsorption of BaP onto soil colloids followed the pseudo-second-order kinetic model (R2 = 0.966), and the adsorption isotherm conformed to the Langmuir model (R2 = 0.995). The BaP and soil colloids primarily formed bonds through π-π interactions and hydrogen bonds. The dynamic experimental outcomes revealed that elevating colloids concentration contributed to increased BaP mobility. Specifically, when the concentration of soil colloids in influent was 500 mg L−1, the mobility of BaP was 23.2 % compared to that without colloids of 13.4 %. Meanwhile, the lowering influent pH value contributed to increased BaP mobility. Specifically, when the influent pH value was 4.0, the mobility of BaP was 30.1 %. The BaP's mobility gradually declined as the initial concentration of BaP in polluted soil increased. Specifically, when the initial concentration of BaP in polluted soil was 5.27 mg kg−1, the mobility of BaP was 39.1 %. This study provides a support for controlling BaP pollution in soil and groundwater.

Mung bean protein colloid mixtures and their fractions - A novel and excellent foam stabiliser

Protein aggregates are known to enhance foam stability by either increasing the thin film viscosity or by blocking the lamellae of a foam. In this study, we produced a mung bean protein colloids mixture (MPCM) by heating protein coacervates that were formed by liquid-liquid phase separation. The functionality of MPCM was compared to a mildly purified mung bean protein extract (MIL). The MPCM showed extraordinary foaming properties, much better than MIL, with foamability of 324 % and a half-life time of 400 min. This work focused on elucidating the exact interface and foam stabilising mechanism of the MPCM by separating the colloids (COL) from the supernatant/continuous phase of the MPCM (SUP). Their interfacial properties were studied by performing surface dilatational rheology and microstructure analysis. Finally, foaming properties, such as foamability, foam stability, and air bubble size, were studied.It was found that the highest foam capacity was observed for the SUP fraction by generating stiffer interfaces. This fraction also contributed to the high foam stability of the MPCM. The COL fraction was found to form a viscoelastic thin film between air bubbles, thereby decreasing the drainage rate of the foam. In brief, SUP and COL fractions co-operate in the formation of highly stable MPCM foam, leading to a promising plant-derived candidate for producing stable foams in food products.

The formation of multi-metal(loid)s contaminated groundwater at smelting site: Critical role of natural colloids

The occurrence and migration of colloids at smelting sites are crucial for the formation of multi-metal(loid)s pollution in groundwater. In this study, the behavior of natural colloids (1 nm-0.45 µm) at an abandoned smelting site was investigated by analyzing groundwater samples filtered through progressively decreasing pore sizes. Smelting activities in this site had negatively impacted the groundwater quality, leading to elevated concentrations of zinc (Zn), lead (Pb), arsenic (As), and cadmium (Cd). The results showed that heavy metal(loid)-bearing colloids were ubiquitous in the groundwater with the larger colloidal fractions (∼75 −450 nm) containing higher abundances of pollutants. It was also observed that the predominant colloids consisted of Zn-Al layered double hydroxide (LDH), sphalerite, kaolinite, and hematite. By employing multiple analytical techniques, including leaching experiments, soil colloid characterization, and Pb stable isotope measurements, the origin of groundwater colloids was successfully traced to the topsoil colloids. Most notably, our findings highlighted the increased risk of heavy metal(loid)s migration from polluted soils into adjacent sites through the groundwater because of colloid-mediated transport of contaminants. This field-scale investigation provides valuable insights into the geochemical processes governing heavy metal(loid) behavior as well as offering pollution remediation strategies specifically tailored for contaminated groundwater.

Humic acid enhances phosphorus transport in soil and uptake by maize

AbstractBackgroundMost of the phosphorus (P) applied to low fertility, acidic tropical soils with high iron and aluminum oxide contents end up adsorbed to soil colloids and not available to crops. Diffusion of P from fertilizers was found to be facilitated by coating with humic substances and by soil moisture.AimHowever, there is still controversy on the effect of humic substances on soil P forms and diffusion, and there is a gap in the knowledge of how coating the fertilizer interacts with soil water.MethodsWe studied P diffusion and availability from a conventional single superphosphate and a humic acid‐coated superphosphate in Petry dishes as affected by soil moisture and the effect of the distance of the fertilizer to the roots using a root‐matt methodology.ResultsCoating single superphosphate with humic substances from leonardite (0.5%) and increasing water moisture resulted in improved P diffusion in the soil and plant P uptake. Maize dry matter increased by humic acid‐coated phosphate from 3% to 26%, depending on the distance from the fertilizer to the roots. Therefore, there is evidence of a lower P adsorption to soil colloids. The coated fertilizer increased labile P by 76% around the fertilizer granule, while higher levels of less labile P were observed with the use of conventional single superphosphate.ConclusionAll this resulted in facilitated P transport from the soil to the roots, which is important mainly in double‐cropped systems where the second crop is usually exposed to temporary drought.

Synergistic effects of organic carbon and silica in preserving structural stability of drying soils

Predicted climate warming and prolonged droughts pose a threat to the soil structure as organic carbon losses weaken the stability of soil aggregates. Well-structured soils are important for storage and movement of water, solutes, and air, the development of plant roots, as habitat for soil organisms, and the microbial activity. Structural stability is measured in terms of hydro-mechanical properties. This study compares effects of amorphous silica with those of organic carbon on stability parameters during drying of aggregates from relatively finer- and coarser-textured soils. Silica amendment enhanced the positive effect of organic carbon on structural stability in terms of the tensile strength. Synergistic effects between silica and organic carbon in soil colloids appear to dynamically alter aggregate density and friability (i.e., ability to crumble) during drying. Silica together with organic carbon could help soil management to reduce negative effects of predicted prolonged droughts on soil structure and stability.

Superabsorbent Hydrogels in the Agriculture and Reclamation of Degraded Areas

Superabsorbent materials (SAPs) are crosslinked polymer networks composed of ionic and non-ionic monomers. SAPs can absorb and retain water solutions up to several hundred times their own weight. As a result of swelling, they form a gel that is insoluble in water, considered safe and decomposes over time. This review focuses on the synthesis, degradation and chemical composition of SAP materials, with particular emphasis on chemical substances that are soluble in water and can migrate into the environment. Numerous applications of natural and synthetic hydrogels in agriculture and the reclamation of degraded areas in preventing erosion, retention water, reducing leaching of colloidal soil components and plant protection products, fertilisers and mineral salts into surface waters have been described. The influence of SAPs on the microbial activity of soils is described. New trends in the search for environmentally friendly SAPs made of modified biopolymers and waste materials are presented, which not only increase yields, but also ensure sustainable agro-environmental development.

A screening model for predicting the potential of soil colloids-enhanced leaching of hydrophobic organic contaminants to groundwater at contaminated sites

Modeling the fate and transport of organic pollutants at contaminated sites is critical for risk assessment and management practices, such as establishing realistic cleanup standards or remediation endpoints. Against the conventional wisdom that highly hydrophobic persistent organic pollutants (POPs) (e.g., polybrominated diphenyl ethers and polycyclic aromatic hydrocarbons) in surface soils are essentially immobile, mounting evidence has demonstrated the potential of these contaminants leaching into the groundwater, due to enhanced transport by soil colloids. Here, we develop a Colloids-Enhanced Transport (CET) model, which can be used as a simple screening tool to predict the leaching potential of POPs into groundwater, as mediated by soil colloids. The CET model incorporates several processes, including the release of POPs-bearing colloids into the porewater, the vertical transport of colloids and associated POPs in the vadose zone, the mixing of POPs-containing soil leachate with groundwater, and the migration of POPs-bearing colloids in saturated zone. Thus, using parameters that can be easily obtained (e.g., annual rainfall, soil type, and common hydrogeological properties of the subsurface porous media), the CET model can estimate the concentrations of POPs in the saturated zone from the observed POPs concentrations in surface or shallow subsurface zones. The CET model can also be used to derive soil quality standards or cleanup endpoints by back-calculating soil concentrations based on groundwater protection limits.

Effect of alkali residue and humic acid on aggregate structure of saline‐alkali soil

AbstractHumic acid (HA) and alkali residue (AR) exert an essential impact on soil aggregate structure, but how HA and AR promote the formation of macroaggregates jointly is not clear. The aim of this research was to explore influence of alkali residue and humic acid (AHA) on the structure and stability of soil aggregates, with saline‐alkali soil in North China as an example. The morphology and structure of the soil with AHA added were detected by a scanning electron microscope, Fourier transform infrared spectroscopy, and X‐ray diffraction. Soil aggregate structure was analyzed by mean weight diameter (MWD), geometric mean diameter (GMD), and fractal dimension (D), and the trend of soil colloid forming macroaggregates was evaluated by zeta potential and surface charge. The results showed that under dry and wet sieves, MWD and GMD both surged initially and then decreased with the rise of AHA content, while D had the opposite trend. With the addition of 3%, AHA, MWD, and GMD reached their peak which proves that the proportion of soil macroaggregates was the highest. In the meantime, absolute value of zeta potential was low, which illustrates that the soil colloid system was in an unstable state of condensation and microaggregates tend to transform into macroaggregates. A large number of ions such as Ca2+ and Mg2+ in AHA promoted variable charge of saline‐alkali soil, and the binding frequency of Ca2+ with negatively charged humus and soil colloid particles was increased. AHA holds great promise for improving the aggregate structure of saline‐alkali soil.

Soil colloids as binding agents in the formation of soil microaggregates in wet-dry cycles: A case study for arable Luvisols under different management

In the hierarchical model of soil aggregates, small soil microaggregates (small SMA; <20 μm) are often considered to be fundamental building units at the micron scale. Below which, soil colloids (<1 µm) have recently been proposed as binding agents of (micro)aggregates. However, the way in which soil colloids contribute to the formation and stability of soil micro- and macroaggregates remains largely unknown. For clarification, we evaluated potential impacts of the colloidal content, particularly the <450 nm colloids, on the aggregation of small SMA. Free water stable small SMA and <450 nm colloids were isolated from Ap-horizons of Stagnic Luvisols under different management (cropped and bare fallow). The size-resolved elemental composition of the <450 nm colloids was analyzed by asymmetric flow field-flow fractionation in combination with an inductively coupled plasma mass spectrometer and an organic carbon detector. To vary the colloidal content in small SMA, (1) suspensions containing different amounts of <450 nm colloids were added in small SMA, or (2) <1 µm colloids were removed from small SMA by centrifugation. In the maximum colloidal addition treatment, the mass ratios of added colloids to small SMA were 3.0 and 5.1 wt% for the cropped and bare fallow soil samples, respectively. Aggregation of small SMA with different colloidal amounts was performed in three successive wet-dry cycles. Afterwards, the size distribution of the resulting aggregates was measured by laser diffraction. Our results indicated that, in wet-dry cycles, colloids were important binding agents for the formation of SMA. Their presence, especially those <450 nm, was likely to support the formation of solid bridges during drying at particle contacts of 1–10 µm small SMA, favoring hereby SMA build-up in a relatively small size range of 1–40 µm. In contrast, the absence of <1 μm colloids in small SMA led to a preferential generation of relatively large aggregates in wet-dry cycles, i.e., typically with sizes >40 μm up to 1700 μm in maximum. Our study on aggregation in wet-dry cycles revealed that the colloidal content has a controlling effect on the size distribution of resulting aggregates by acting as a binding agent and provides hereby new insights into the evolvement of aggregate hierarchy in soils.

Influence of Soil Colloids on the Transport of Cd2+ and Pb2+ under Different pH and Ionic Strength Conditions

The co-transport of contaminants by soil colloids can generate substantial environmental risk, and this behavior is greatly affected by environmental conditions. In this study, AF4-ICP-MS was used to investigate the size distribution and composition of Cd/Pb-bearing colloids; saturated sand column experiments were used to investigate the impact of soil colloids on the transport of Cd/Pb under different pH and ionic strength conditions. AF4-ICP-MS characterization showed that natural colloids were primarily associated with two sizes ranges: 0.3–35 KDa (F1, fine nanoparticles) and 280 KDa–450 nm (F2, larger nanoparticles), which mainly consisted of organic matter (OM), iron (Fe), and manganese (Mn) (oxy)hydroxides and clay minerals. Fine nanoparticles could strongly adsorb Cd and Pb under all environmental conditions. Mn and Fe (oxy)hydroxides generally formed under neutral to alkaline conditions and exhibited adsorption capabilities for Cd and Pb, respectively. Transport experiments were conducted under different pH and ionic strength conditions. At pH 3.0, soil colloids had little effect on the transport of Cd2+ and Pb2+. At pH 5.0, soil colloids inhibited the transport of Cd2+ by 16.1%, and Pb2+ recovery was still 0.0%. At pH 7.0 and 9.0, soil colloids facilitated the transport of Cd2+ by 15.6% and 29.6%, facilitated Pb2+ by 1.3% and 6.4%. At an ionic strength of 0, 0.005, and 0.01 mol L−1 NaNO3, soil colloids facilitated the transport of Cd2+ by 77.7%, 45.8%, and 15.6%, only facilitated the transport of Pb2+ by 46.2% at an ionic strength of 0 mol L−1 NaNO3. At an ionic strength of 0.05 mol L−1 NaNO3, soil colloids inhibited the transport of Cd2+ and Pb2+ by 33.1% and 21.0%, respectively. The transport of Cd2+ and Pb2+ facilitated by soil colloids was clearly observed under low ionic strength and non-acidic conditions, which can generate a potential environmental risk.