Geochemical Properties of Variable Charge Soil Explain the Low Nitrogen Bioavailability

Surface charge characteristics were investigated using a charge fingerprinting procedure for 90 samples from 32 profiles of highly weathered Oxisols and Ultisols derived from shale/limestone, basalt, granite, local alluvium, sedimentary, and metasedimentary rocks under tropical savanna and tropical monsoonal climates in Thailand. The charge fingerprints of 5 whole soils, after removal of organic matter and of kaolin and ion oxides from these soils, were also measured to clarify factors contributing to the variable charge behaviour. Phosphate sorption was determined and related to charge properties. Kaolin was the dominant mineral of the clay fraction with moderate amounts of goethite and hematite. Gibbsite was present in Oxisols formed on limestone and basalt under more humid conditions. All soils exhibited variable charge behaviour but the magnitude and rate of change in variable charge with pH varied greatly between Oxisols and Ultisols. Oxisols had higher amounts of variable charge than Ultisols, reflecting the differences in amounts of clay and extractable Fe and Al. Oxisols formed on basalt and limestone under more humid conditions had high values of anion exchange capacity (AEC) due to the contributions of goethite, gibbsite, and boehmite. The relationships of base cation exchange capacity (CECB) and AEC with pH were linear. Rates of change with pH of both negative and positive variable charge (Ac, Aa) were closely related to CEC, specific surface area (SSA), pH(NaF), and various forms of extractable Fe, Al, and Mn. The charge fingerprints of soil after removal of organic matter indicated that organic matter exerted a strong influence on both the magnitude of negative charge and rate of change with pH. Kaolin had permanent and variable charge, with SSA and crystal size (MCD001) being highly predictive of the rate of change in variable negative charge with pH. Iron oxide concentrates exhibited strongly pH-dependent charge and the mean coherently diffracting length (MCD110) of hematite was highly predictive of both the magnitude and rate of change in variable positive charge with pH. Charge coefficients (Ac, Ba) had highly significant (P = 0.005) positive relationships with Langmuir and Freundlich phosphate sorption maximum coefficients (Xm, k), indicating that the surfaces of amorphous, poorly ordered, and crystalline Fe and Al oxides are prime sites for both P sorption and variable charge.

The capacity of biochars to adsorb ionic contaminants is strongly influenced by biochar surface chemistry. We studied the effects of biomass feedstock type, pyrolysis temperature, reaction media pH, and AlCl pre-pyrolysis feedstock treatments on biochar anion exchange capacity (AEC), cation exchange capacity (CEC), point of zero net charge (PZNC), and point of zero salt effect (PZSE). We used the relationship between PZNC and PZSE to probe biochar surfaces for the presence of unstable (hydrolyzable) surface charge functional groups. The results indicate that biochars produced at ≤500°C have high CECs and low AEC, PZSE, and PZNC values due to the dominance of negative surface charge arising from carboxylate and phenolate functional groups. Biochars produced at ≥700°C have low CEC and high AEC, PZSE, and PZNC values, consistent with a dominance of positive surface charge arising from nonhydrolyzable bridging oxonium (oxygen heterocycles) groups. However, biochars produced at moderate temperatures (500-700°C) have high PZSE and low PZNC values, indicating the presence of nonbridging oxonium groups, which are rapidly degraded under alkaline conditions by OH attack on the oxonium α-C. Biochars treated with AlCl have high AEC, PZSE, and PZNC values due to variably charged aluminol groups on biochar surfaces. The results provide support for the presence of both hydrolyzable and nonhydrolyzable oxonium groups on biochar surfaces. They also demonstrate that biochars produced at high pyrolysis temperatures (>700°C) or those receiving pre-pyrolysis treatments with AlCl are optimized for anionic contaminant adsorption, whereas biochars produced at low pyrolysis temperatures (400°C) are optimized for cationic contaminant adsorption.

Callitriche cophocarpa biomass as a potential low-cost biosorbent for trivalent chromium

Salt retention has been observed in many variable charge soils. However, studies on the extent of the nitrate salt retention in tropical and subtropical soils are not documented in the literature. No attempts have been made to relate the extent of nitrate salt retention with soil mineralogy and the parameters of the soil chemistry, such as cation and anion exchange capacities (CEC and AEC) and soil solution pH and ionic strength (IS). Our objective was to provide evidence of the retention of Ca(NO 3 ) 2 in variable charge subsoils with extremely diverse mineralogical and chemical characteristics and to estimate the change in the extent of salt retention as a function of pH and IS. Laboratory leaching experiments were conducted with disturbed subsoil materials collected from the southeastern United States and other areas. Four lime treatments and four Ca(NO 3 ) 2 solutions of different concentrations were used to test the effect of pH and IS on nitrate salt retention. The results show that the transport of Ca(NO 3 ) 2 is affected by soil mineralogy and chemistry. The magnitude of salt retention was higher in subsoils with appreciable AEC and equivalent amounts of CEC, where both kaolinite and Al/Fe oxides dominate the clay fraction. In the subsoils with the highest magnitude, the electrical conductivity breakthrough curves (EC BTC) have an extended plateau of low EC values, and the first pore volumes of leachate contain virtually no salt. The cation and anion of the leaching electrolyte (Ca ++ and NO 3 - ) are adsorbed with no net release of other ions from exchange sites. Both pH and IS of the subsoil solution affect ion transport and alter the extent of Ca(NO 3 ) 2 2 retention. This study leads to the conclusion that mineralogy, surface charge, pH, and IS affect the magnitude of salt retention in variable charge subsoils, and their effects should be considered when predicting ion mobility and transport in these subsoils.

Present investigation aimed to convert rock phosphate ore into nano zeolite using calcination (from 200 to 700°C) for 48 hrs. and zeolitization (replacement by a zeolite mineral) processes in order to boost rock phosphate ore efficiency as a source of P2O5 even under high soil pHs. Crystallization, phases, physico-chemical characteristics and surface morphology were studied by visual techniques. Using scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS) which cleared that, the converted rock phosphate gave different shapes and sizes of crystals and distribution of its components (elements and oxides mass percentage). While TEM appears the crystals size of converted rock phosphate was found in nano size (19.7 -39.1 nm). Moreover Sterio microscopy illustrated that, after calcination and zeolitization processes, the nano zeolite converted from rock phosphate ore took different shapes with three dimension crystals growth as end of crystallization process. Whilst XRD manifested that, the dominant mineral was zeolite associated with different minerals represented by mica muscovite, montmorillonite, calcite, pyrite, alkali feldspar, plagoclase feldspar and quartz. At the same time, XRF was used to verify nano rock phosphate converted in to zeolite (heulandites type). Also the XRF analysis recorded Si/Al ratio (3.42%) of nano zeolite its properties tended to both hydrophilic and organophillic. Furthermore, DTA (differential thermal analysis) and TGA (thermo-gravimetric analysis) were used for measure percentages of water molecules attached with nano rock phosphate and its thermal stability. Therewithal, Surface area (BET) was (14.93m2/g), pore sizes distribution from 3.98 to 56.87 nm, pore volume19.96 nm and pore width 4.846 cm3 / g. It was observed that there is little change in particles density in both real and bulk densities. Finally, the highest CEC (cation exchange capacity) and lowest AEC (anion exchange capacity) values of the conversion rock phosphate were acquired. In a nutshell, our results designated for more liberate of P2O5 in available and safety form to uptake easily by cultivated crops, and construct an appropriate media for loading by beneficial microorganisms.

Enhancing cation and anion exchange capacity of rice straw biochar by chemical modification for increased plant nutrient retention

The amount of chemical inputs influences long-term soil nutrient balance. Therefore, different management practices will determine the sustainability of land use. This study aimed to determine the effects of paddy field management systems on the Soil Fertility Index (SFI), identify the characteristics of clay content and its effects on SFI, and identify the determinants of SFI. The originality of this research lies in identifying the determinants of soil fertility based on soil characteristics, enabling more focused, effective, and efficient land management recommendations. Soil sampling was carried out at 24 points, comprising 8 LMUs, across 3 types of rice field management and slope gradients (8-15%) in Purwantoro District, Wonogiri Regency. Parameters observed included pH, organic carbon (organic C), Total Nitrogen (N), available Phosphorus (P), available Potassium (K), base saturation, cation exchange capacity (CEC), Aluminium (Al) saturation, and clay content. The SFI class was generated by matching data with classification, while the influence of rice field management type on SFI was analyzed using ANOVA and the determining factors of SFI were analyzed using correlation test. The study found that the soil fertility index ranged from 0.33 (low) to 0.50 (moderate). The soil fertility of each type of rice field management varied, with organic rice fields having the highest soil fertility (0.48c), semi-organic rice fields (0.42c), and conventional rice fields (0.36a). Soil fertility was also related to the proportion of clay (r2=0.2112). The strongest factors determining soil fertility were Organic Carbon (0.905) and CEC (0.805).

Variable-charge (v-c) and permanent-charge (p-c) soils differ fundamentally with regard to many nutrient-cycling processes. Variable-charge soils are more common in the tropics than in temperature zones because their formation requires desilication, which proceeds fastest in warm, moist climates. The dynamics of nutrient mobility tend to be more complex in v-c than in p-c soils. For example, theory predicts that, as pH of v-c soils decreases, cation exchange capacity (CEC) also decreases and anion exchange capacity (AEC) increases. If AEC exceeds CEC, cations such as ammonium and potassium will be more mobile than anions such as nitrate; this is the reverse of the situation in p-c soils, on which most of our knowledge of nutrient cycling is based. Variable-charge surfaces sorb phosphorus, creating plant nutritional problems throughout large areas of the humid tropics. Desilication, the same process that creates v-c surfaces, results also in stable aggregation, creating soils that retain water, yet drain rapidly and resist erosion. The Soil Taxonomy system incorporates information on mineralogy, texture, and organic matter content, and therefore provides insights into patterns of charge chemistry and nutrient cycling across a wide range of soil types.

It has been demonstrated that electric regeneration of so‐called bipolar electrolyte exchange resins into the H+OH− form may take place between permselective ion‐exchange membranes provided that the resin was placed in an aqueous medium. The level of regeneration of cation and anion capacities depends on the ratio of the amount of cationic and anionic active groups \documentclass{article}\pagestyle{empty}\begin{document}$ \[\underline{\underline + } \] $\end{document} index in the resin. In resins of ± > 1 type the cation exchange capacity in H+ form—expressed in percentage of total cation exchange capacity—is constantly higher than the anion exchange capacity in OH− form, /expressed in the same way/ while in the case of \documentclass{article}\pagestyle{empty}\begin{document}$ \[\underline{\underline + } > 1\] $\end{document} < 1 type resins the situation is inversed. An explanation has been given for the mechanism of this phenomenon. A method has been elaborated for the determination of H+ and OH− capacities simultaneously.

Biochar can be an effective sorbent material for removal of nutrients from water due to its high specific surface area, porous structure, and high cation and anion exchange capacity. The aim of this study was to test a biochar reactor and to evaluate its efficiency in runoff water purification and consecutive nutrient recycling in clear-cut peatland forests. The goodness of the method was tested in a meso-scale (water volume thousands of liters) reactor experiment by circulating runoff water through wood biochar-filled columns and by determining water nutrient concentrations in the column inlet and outlet. The pseudo-first and second order kinetic models were fitted to the experimental data and the adsorption rate (Kad) and maximum adsorption capacity (Qmax) of the biochar reactor were quantified. The concentration of total nitrogen (TN) decreased by 58% during the 8-week experiment; the majority of TN adsorption occurred within the first 3 days. In addition, NO3-N and NH4-N concentrations decreased below the detection limit in 5 days after the beginning of the experiment. The maximum adsorption capacity of the biochar reactor varied between 0.03–0.04 mg g−1 biochar for NH4-N, and was equal to 0.02 mg g−1 biochar for TN. The results demonstrated that the biochar reactor was not able to adsorb TN when the water TN concentration was below 0.4 mg L−1. These results suggest that a biochar reactor can be a useful and effective method for runoff water purification in clear-cut forests and further development and testing is warranted. Unlike traditional water protection methods in peatland forestry, the biochar reactor can effectively remove NO3-N from water. This makes the biochar reactor a promising water protection tool to be tested in sites where there is the risk of a high rate of nutrient export after forest harvesting or drainage.

Alteration of Alumina Pillared Clays for Enhanced Catalytic Activity

Relationships between cation exchange capacity (CEC), clay and organic carbon contents and soil pH were analyzed by way of multiple regressions for upland soils in eastern Canada (mostly Ontario, with additional data for New Brunswick). This was done by vegetation type in an attempt to explain some of the otherwise unexplained CEC variations. Data were taken from about 2000 soil horizons (organic L, F, and H horizons as well as A, B, and C mineral soil horizons) under broadleaves (mostly maples, beech, birch or aspen as dominant species), conifers (mostly fir, spruces and/or pines), and grass vegetation. For the organic forest floor horizons (or L, F, and H horizons), both organic carbon content (%) and pH were highly significant for predicting CEC, i.e.,CEC (L, F, and H of broadleaves) = −38 + 0.71 × org. C (%) + 10.3 × pH (R2 = 0.69), andCEC (L, F and H of conifers) = −31 + 0.34 × org. C (%) + 12.1 × pH (R2 = 0.58).For the mineral soil, clay and organic carbon contents (%) and pH were highly significant for predicting CEC. Soils with forest vegetation were found to have lower contributions of organic matter to CEC than grassland soils, i.e.,CEC (forest soils) = −7.0 + 0.29 × clay (%) + 0.82 × org. C (%) + 1.4 × pH (R2 = 0.72),CEC (wooded grasslands) = −6.0 + 0.31 × clay (%) + 1.31 × org. C (%) + 1.0 pH (R2 = 0.74), andCEC (grasslands) = −8.3 + 0.24 × clay (%) + 2.14 × org. C (%) + 1.3 × pH (R2 = 0.79).Relationships that were developed from Ontario data for specific vegetational types (maple sites, strongly podzolized conifer sites, grasslands/croplands) were tested by comparing CEC predictions with reported values for similar sites in New Brunswick and Quebec. The predictions were consistent with the general trends for maple sites and grasslands/croplands, but CEC values were strongly overpredicted for Podzolic subsoils on conifer sites.Literature information of the CEC dependency on in situ pH is sparse. Existing information that is based on buffering grassland/cropland soil samples from pH 2.5 to 8 appears to mimic this dependency quite well. Key words: Cation exchange capacity, clay, organic carbon, soil pH, forests, grasslands

Understanding the factors that govern arsenic mobility in soils is essential for effective risk assessment and remediation. Despite existing knowledge, a global-scale synthesis linking soil properties to arsenic mobility across diverse texture classes remains limited. This study addresses this gap by using a globally harmonized dataset and machine learning (ML) models, the combined influence of soil pH, organic carbon (OC), cation exchange capacity, total arsenic and clay content on the mobile arsenic fraction (PF₁) has been modelled. Random Forest, XGBoost, and Generalized Additive Models were trained to predict PF₁, with Random Forest outperforming others (R² = 0.65, RMSE = 13.21 and MAE = 6.93). Simulation involving 25,200 data points across 12 USDA texture classes revealed pronounced non-linear interactions between pH and OC, significantly affecting PF₁ values. Arsenic mobility was found to be lowest under slightly acidic pH (5.5-6.5) and low-to-moderate OC levels (< 2%), particularly in clay-rich soils. Conversely, coarse-textured soils exhibited broader “mobility control zones” where PF₁ remained below the 10% risk threshold, while fine-textured soils were more sensitive to pH and OC fluctuations. Kernel density estimations further delineated typical OC-pH combinations associated with low arsenic mobility. Although redox conditions and mineralogical influences were beyond the scope of this study, the findings underscore the importance of incorporating physicochemical parameters and soil texture into arsenic risk frameworks. Attempts of reducing arsenic concentrations through amendments or stabilization, management practices should also account for soil texture and chemistry, ensuring interventions are tailored to minimize the fraction of arsenic that remains mobile. Graphical Abstract This graphical abstract provides a concise, visually engaging summary of the research, enabling readers to rapidly grasp the study’s key insights without delving into the full manuscript. The central focus of the study is the prediction of arsenic (As) mobility in soils using global harmonized datasets incorporating critical soil attributes such as organic carbon (OC), soil pH, cation exchange capacity, clay content, total arsenic in soil, and mobile arsenic fraction (PF₁). These inputs feed into machine learning and statistical modeling approaches-Generalized Additive Models , XGBoost, and Random Forest-to assess and predict arsenic behaviour across diverse soil systems. Model performance indicators (R², RMSE, and MAE) are displayed for comparative evaluation, highlighting model efficiency. At the core of the abstract, a globe symbolizes the global relevance of the harmonized data, while simulation outputs illustrate 25,200 data points across 12 soil texture classes to evaluate OC and pH effects. The non-linear 3D surface plot emphasizes complex relationships between arsenic fraction, OC, and pH, reinforcing the need for advanced modelling. Furthermore, the lower left quadrant illustrates that fine-textured soils exhibit greater sensitivity to pH and OC fluctuations, while coarse-textured soils with high OC content tend to enhance arsenic mobility. This visualization underscores the importance of considering soil type and chemical properties in environmental risk assessments. Collectively, the graphical abstract encapsulates the study’s methodology, findings, and implications for managing arsenic contamination in agricultural soils at a global scale.

Fundamental studies of novel inorganic–organic charged zwitterionic hybrids: 3. New hybrid charged mosaic membranes prepared by modified metal alkoxide and zwitterionic process

Ion-exchange properties and thermal stability were studied for hydrous TiIV–ZrIV oxide ion exchangers with various contents of titanium. The cation-exchange capacity increased with the titanium content in the exchanger, along with a decrease in the anion-exchange capacity. Though the affinity for representative metal ions increases with the acidity of the exchange sites, no such simple relation holds for transition metal ions. Exchangers with 30 to 70 mol% Ti (the mol% of Ti to Ti+Zr) do not suffer from drastic changes in both the water content and crystallinity upon heating to 500°C; they are much more heat-resistant than component simple hydrous metal oxides. Though the total cation-exchange capacity decreases with increasing temperature during a heat treatment, only a minute amount of the acidic sites dissociating at a pH lower than the isoelectric point of the exchangers with ≤70 mol% Ti increases upon heating. The effect of a heat treatment on the distribution coefficients of cations has been explain...