Soil Adsorption Research Articles

Continuous planting Eucalyptus plantations in subtropical China: Soil phenolic acid accumulation and adsorption physiognomies

As phenolic acids accumulated in monocropping plantations, their allelopathic effects caused the soil’s fertility to decline and productivity to decrease. The effects of stand age and generational turnover of phenolic acids in soil remain unclear. Therefore, we conducted an experiment to examine the changes in phenolic acid content and accumulation characteristics in soils from Eucalyptus plantations in terms of planting years and generations. Soils were collected from three Eucalyptus plantations: a 6-year-old first-generation plantation (6a1g), 9-year-old first-generation plantation (9a1g) and 6-year-old second-generation plantation (6a2g). Seven phenolic acids were identified from different soil samples by high-performance liquid chromatography. In addition, exogenous phenolic acids were added to the soil samples to study the kinetics as well as the adsorption and desorption characteristics of soils. The total of seven phenolic compounds in 6a1g (38.451 μg g–1) was higher than that of the 9a1g and 6a2g soils, and the total of 6a2g (27.257 μg g–1) was higher than that of the 9a1g (15.536 μg g–1). In the tested soil, the p-hydroxybenzoic acid accounted for 51.6% (for 6a1g), 51.1% (for 9a1g) and 33.5% (for 6a2g) of the total amount of the seven phenolic acids, the p-hydroxybenzoic may have had a higher allelopathic capacity in these Eucalyptus plantation soils. Soil bulk density showed a strong positive correlation with vanillic and ferulic acids, and SOM also had an extremely positive correlation with p-hydroxybenzoic and coumalic acids. The findings demonstrated that the adsorption and desorption rates and amounts of each phenolic acid in the soil of 6a1g were higher than those in the soil of 9a2g and 6a2g using both a kinetic adsorption experiment and isothermal adsorption and desorption experiments. But among the three soils, 9a1g soil had the lowest adsorption rate, which resulted in the lowest accumulation capacity. It is noticed that the Pseudo-second-order kinetic model can be used to describe the kinetic adsorption better than the Pseudo-first-order kinetic model and the Elovich model, and the Freundlich model could fit the desorption isotherms better than the adsorption isotherms. However, more study of this phenomenon is essential to better understanding its mechanisms and how they function in order to address issues with soil degradation in future intergenerational Eucalyptus plantings.

Influences of microplastics types and size on soil properties and cadmium adsorption in paddy soil after one rice season

Influences of microplastics types and size on soil properties and cadmium adsorption in paddy soil after one rice season

The Mechanism for the Barrier of Lunar Regolith on the Migration of Water Molecules

AbstractThis paper reports the study of potential water retention mechanisms based on the migration process of water molecules in the lunar regolith. First, we propose innovative definitions for the adsorption barrier coefficient, the collision barrier coefficient, and the total barrier coefficient for water molecule migration in the lunar regolith. Second, we derive an equation that relates the different barrier coefficients. Finally, we develop an equation for the diffusion coefficient of water in the regolith. The results are as follows. The collision barrier coefficient is a function of the soil layer porosity and pore number density and increases with increasing pore number density and decreasing porosity. The adsorption barrier coefficient depends on the soil temperature, adsorption heat, porosity, and pore number density and decreases sharply with increasing temperature. The adsorptive capacity of lunar soil to adsorb water molecules increases with increasing the adsorption barrier coefficient. For soil layers with a lower temperature, adsorption plays a more important role than collision in hindering the migration of molecules in the regolith. The total barrier coefficient increases with decreasing temperature, increasing adsorption heat, decreasing porosity, and increasing pore number density. Our results suggest that the water loss rate is inversely proportional to the total barrier coefficient. The water loss rate decreases with decreasing soil temperature, increasing adsorption heat, decreasing porosity, and increasing pore number density. Namely, water is more likely to be retained in areas where the regolith has a low temperature, large adsorption heat, and both low porosity and large pore number density.

Interactions of Microplastics with Pesticides in Soils and Their Ecotoxicological Implications

In the middle of the 20th century, the production of plastics exploded worldwide because of their low cost and the versatility of their applications. However, since plastic debris is highly resistant to environmental degradation, a growing presence of plastics in all the ecosystems has been confirmed. Among them, plastic particles < 5 mm, also known as microplastics (MPs), are of special concern because they are dispersed in aerial, terrestrial and aquatic environments, being the soil the main environmental sink of these contaminants. Due to their large specific surface area and hydrophobicity, MPs are considered good adsorbents for other environmental organic pollutants also present in terrestrial ecosystems, such as pharmaceuticals, personal-care products or pesticides with which they can interact and thus modify their environmental fate. In this review article, we examine the recent literature (from 2017 to 2022) to get a better understanding of the environmental fate of pesticides in soil (adsorption, mobility and/or degradation) when they are simultaneously present with MPs and the ecological risks on living organisms of the interactions between MPs and pesticides in soil. More studies are needed to fully understand the toxicological impact of the copresence in soil of pesticides and MPs.

Soil properties and functional genes in nitrogen removal process of bioretention

ABSTRACT The effects of different soil properties on hydrology and nitrogen removal were studied in a simulated bioretention system. Soil capacity of permeability and water retention, changes in the soil environment, leachate concentrations at the surface and bottom layers, quantification of N removal from soil, microorganism and plant by 15N isotope tracer technique, and functional genes abundance at different depths were evaluated. The results showed that shallow root plants, soil compaction, and low organic matter content were not conducive to the infiltration of bioretention systems. In the 72 h experiment, compaction (especially surface compaction) and planting of herbaceous plants (Ophiopogon japonicus) were not beneficial to the removal of TN, TP, and COD. Adding an appropriate amount of organic matter also affects nitrogen and phosphorus removal. In the process of denitrification, the order of the ability to remove nitrogen is soil adsorption > microbial assimilation > plant uptake. The contribution of soil denitrification is affected by soil compaction, compaction location, plant species and organic matter content. The abundance of 16S rRNA, nitrifying, denitrifying and nrfA genes decreased with soil depth. More copies of genes in topsoil were thought to be due to sufficient nutrients, aerobic condition, anaerobic microsites and submerged state. Soil compaction, organic matter content and plant species affected nitrification, denitrification and DNRA gene characteristics.

Remediation of PAH-contaminated soil using a combined process of soil washing and adsorption by nano iron oxide/granular activated carbon composite

This study investigated the application of magnetic granular activated carbon (MGAC) in soil washing process for the removal of polycyclic aromatic hydrocarbons (PAHs) from real contaminated soil. The magnetic composite was prepared using a facile-modified co-precipitation method. The presence of the synthesized magnetite (Fe3O4) nanoparticles on the composite surface was confirmed by the SEM and XRD characterization tests. The surface area and total pore volume of MGAC were determined to be 837.9 m2/g and 0.5 cm3/g, respectively, which were approximately 10% smaller than those for the bare GAC. The Langmuir model fitted the adsorption isotherm data the best; therefore, monolayer adsorption was predominant in the treatment process with MGAC. The maximum PAH adsorption capacity of MGAC was obtained at 153.5 µg/mg for the total PAHs, which was nearly 20% lower than that of bare GAC (190.3 µg/mg). The soil washing parameters were optimized, and the results demonstrated that the total PAH removal reached a maximum of 47.4% at an MGAC dose of 2% (w/w), washing time of 24 h, a liquid-to-soil ratio of 15:1, stirring speed of 100 rpm, pH of 8.3, and a temperature of 25 °C. For low molecular weight (LMW) PAHs, the removal from soil ranged from 34.6% to 57.7%, whereas the removal percentage of high molecular weight (HMW) PAHs was determined to be between 27.9 and 67.1 %. This indicated that the optimized soil washing with MGAC had a better efficiency for some HMW PAHs than the LMW PAHs. According to the thermodynamic studies, the adsorption of PAHs onto MGAC in the soil washing process was non-spontaneous (ΔG° > 0) and endothermic (ΔH° > 0) with increased randomness at the solid/solution interface of MGAC as temperature increases. The obtained results confirmed the efficiency of the magnetized activated carbon as adsorbent in soil washing to remove the hazardous PAH compounds from soil.

A Superstructure Mixed-Integer Nonlinear Programming Optimization for the Optimal Processing Pathway Selection of Sludge-to-Energy Technologies

The perception of sewage sludge has increasingly changed from being a waste, which is a burden to the environment and society, to a useful resource of materials and renewable energy. There are several available technologies at different stages of maturity that aim to convert sludge to energy in the form of electricity and/or fuels. In this paper, a decision-making support tool is proposed to help in choosing the optimal pathway for the sludge-to-energy conversion from a techno-economic perspective. The conversion technologies under study are: (1) anaerobic digestion, (2) pyrolysis, (3) gasification, (4) incineration, (5) supercritical water oxidation, (6) supercritical water gasification, as well as the corresponding dewatering and drying methods for each technology. Different synergies between the available technologies are compared by the formulation of a superstructure optimization problem expressed in a mixed-integer non-linear program (MINLP) model. The applicability of the proposed model is explored via a case study for a hypothetical sludge treatment plant with a capacity of 100 tons of dry solids (tDS) per day. The model is solved via the BARON solver using GAMS software within a reasonable processing time. According to the obtained results, the fast pyrolysis technology, coupled with filter press dewatering and thermal drying as pre-treatment steps, show the most promising outcomes with the minimum treatment cost of USD 180/tDS. Fast pyrolysis converts the sludge to bio-oil, which can be used as an alternative fuel after further refining, and biochar, which can be used for soil amendment or adsorption purposes. The model parameters are subject to uncertainty that is addressed in the sensitivity analysis section of this paper. Moreover, the pyrolysis pathway shows a high degree of robustness in most of the sensitivity analysis scenarios. Meanwhile, anaerobic digestion coupled with fast pyrolysis demonstrates the best energy recovery performance upon increasing electricity prices.

Assessment of the acute toxicity of agrochemicals on earthworm (Aporrectodea caliginosa) using filter paper contact and soil mixing tests

Abstract. El-Aswad AF, Fouad MR, Aly MI. 2023. Assessment of the acute toxicity of agrochemicals on earthworm (Aporrectodea caliginosa) using filter paper contact and soil mixing tests. Asian J Agric 7: 14-19. Earthworms are suitable bioindicators of chemical contamination in terrestrial ecosystems. Therefore, adult earthworms (Aporrectodea caliginosa Savigny, 1826), which are common in Egyptian rice fields were used to study the side effects of fenitrothion and thiobencarb. Also, it was used two common Egyptian soil types; alluvial soil (clay soil) and the calcareous soil (sandy clay loam soil). Two tests were used; filter paper contact test at 24, 48 and 72 h, and the soil mixing test at 5 and 10 days. The effect of both pesticides on mortality of earthworm was insignificant at 24 h with a low mortality percentage. LC50 value of fenitrothion and thiobencarb were 83.16, 288.26, 24.67, and 39.98 µg a.i mL-1 at 48 and 72 h, respectively. Based on the LC50 values, fenitrothion was 3.5 and 1.6 times more toxic than thiobencarb at 48 and 72 h, respectively. At 5 and 10 days after treatment, respectively, the LC50 for fenitrothion in the soil mixing test decreased from 334.27 to 55.45 in clay soil and from (415.90) to (25.00) in sandy soil. Regarding to fenitrothion in soil mixing test, the LC50 was reduced from 334.27 to 55.45 in clay soil and from 415.90 to 25.00 in sandy soil at 5 and 10 days after treatment, respectively. Also, the LC50 of thiobencarb in clay soil was decreased from 0.93 to 0.41 and from 55.28 to 10.65 at 5 and 10 days, respectively. In general, a positive relation was recorded between the tested pesticide toxicity on earthworm and concentrations, and also exposure time. Fenitrothion was more toxic on earthworm in contact filter paper test than thiobencarb. While, in clay soil, fenitrothion was least toxic to the earthworm in soil mixing test, this could be attributed to the slow degradation in the worms and subsequently less elimination of the metabolites as well as attributed to its higher adsorption and lower desorption in soil.

Organic Carbon Controls Mercury Distribution and Storage in the Surface Soils of the Water-Level-Fluctuation Zone in the Three Gorges Reservoir Region, China.

The particular condition of the water-level-fluctuation zone (WLFZ) in the Three Gorges Reservoir (TGR), the largest hydroelectric reservoir in China, raises great concerns about mercury (Hg) contamination and ecological risk. In addition, previous research found that soil organic carbon (SOC) plays an essential role in controlling Hg distribution and speciation. However, there is minimal information on the Hg storage distribution and their relationships with SOC in the WLFZ in TGR. This study investigated Hg distribution, storage, and their relationships with SOC in the surface soils in WLFZ. The results showed that the total Hg (THg) content in the surface soils ranged from 18.40 to 218.50 ng g-1, with an average value of 78.17 ± 41.92 ng g-1. About 89% of samples had THg content above the background value in Chongqing, showing specific enrichment of Hg in WLFZ due to contamination in the TGR. The surface soils have low SOC, with an average value of 8.10 ± 3.90 g kg-1. Moreover, THg content showed consistent distribution with the SOC in WLFZ, with a significantly positive correlation (R = 0.52, p < 0.01, n = 242). THg storage (201.82 ± 103.46 g ha-1) in the surface soils was also significantly positively correlated with the SOC storage (R = 0.47, p < 0.01, n = 242). The reduced SOC sequestration, due to the periodical alternative "flooding-draining" and frequent reclamation and utilization of WLFZ, decreased the Hg adsorption in soil. Those might result in the re-release of Hg into waters when WLFZ is flooded. Therefore, more attention should be directed towards Hg cycling and the consequent environmental risks in the TGR region.

Modeling fate and transport of pesticides from dryland agriculture using SWAT model

The aim of this study was to understand pesticide fate and transport from dryland agriculture in a major drinking water basin using SWAT and to identify critical source areas in the basin. Hydrological calibration results indicated satisfactory simulation of hydrologic processes within the catchment. Long term average observed sediment values (0.16 ton/ha) were compared with the annual average simulated SWAT outputs (0.22 ton/ha). Generally, the simulated concentrations were higher than the observed values, but the distribution pattern and trends were similar among the months. Average concentrations in water were 0.036 μg/L and 0.006 μg/L for fenpropimorph and chlorpyrifos, respectively. Transfer rates of pesticides from landscape to rivers showed that 0.36% of fenpropimorph and 0.19% of the applied amount of chlorpyrifos were exported to the river. Higher amount of fenpropimorph transport from land to the reach was attributed to its lower Koc (soil adsorption coefficient) value compared to chlorpyrifos. Higher amounts from HRUs were observed in the application month (April) and following month (May) for fenpropimorph, while the months after September showed higher amounts for chlorpyrifos. The specific HRUs (Hydrological Response Units) located in sub-basins 3, 5, 9 and 11 presented highest dissolved pesticide amounts, while HRUs in sub-basins 4 and 11 exhibited highest concentrations for adsorbed pesticides. Best management practices (BMPs) were recommended in critical subbasins to protect the watershed. Despite the limitations, the results demonstrate the potential contributions of modeling in terms of assessing pesticide loadings, critical zones and application timing.

Transport and fate of Cu and Cd in contaminated paddy soil under acid mine drainage

Long-term heavy metals accumulation caused by acid mine drainage (AMD) irrigation in paddy soils poses a severe threat to environmental health. However, the soil adsorption mechanisms under AMD flooding remain unclear. This study provides key insights into the fate of heavy metals in soil, particularly the retention and mobility mechanisms of Cu (copper) and Cd (cadmium) after AMD flooding. The migration and fate of Cu and Cd in uncontaminated paddy soils treated by AMD in Dabaoshan Mining area was investigated via column leaching experiments in the laboratory. The maximum adsorption capacities of Cu (658.04 mg kg−1) and Cd (335.20 mg kg−1) cations were predicted and the breakthrough curves were fitted using the Thomas and Yoon-Nelson models. Our findings demonstrated that Cd was more mobile than Cu. Furthermore, the soil had a greater adsorption capacity for Cu than Cd. Tessier's five-step extraction method was used to determine Cu and Cd fractions in leached soils at different depths and times. After AMD leaching, the relative and absolute concentrations of the easily mobile forms were all increased at different soil depths, thus increasing the potential risk to the groundwater system. Mineralogical characterization of the soil indicated that AMD flooding leads to the formation of mackinawite. This study provides insights into the distribution and transportation processes of soil Cu and Cd and their ecological effects under AMD flooding, as well as a theoretical basis for the establishment of corresponding geochemical evolution models and environmental governance in mining areas.

Development of a simple test method for inhibitory effect of gall formation on Meloidogyne incognita and its application to nematicide screening

Summary For a screening test to discover nematicide candidates to control Meloidogyne incognita, a potted plant-based assay to observe gall formation on the root is realistic because it can cover the nematode life-cycle period and a broad range of mode of actions of tested compounds. However, both time and labour are needed for its operation. The objective of this study was to develop a plant-based screening method enabling easy periodical observation of the progress of root gall formation under almost no soil adsorption conditions of tested compounds. As an alternative material for soil, rockwool substrate for growing vegetable seedlings was used with a nutrient solution. Observation of gall formation on the same substrates was possible for 3-4 weeks. With this method, standard nematicides, e.g., fosthiazate and fluopyram, provided clear inhibitory effect on gall formation. Their ED50 values at 21 days after inoculation of 300 second-stage juveniles (substrate)−1 were 0.0034 mg (substrate)−1 and 0.0035 mg (substrate)−1, respectively. Furthermore, conventional fungicides and insecticides were screened with the method. Activities were observed in some products like polyoxins, which showed 100% control at 25 mg (substrate)−1, a considerably higher dose than that in standard nematicides. This study developed a simple test method for evaluating inhibitory effect on gall formation by M. incognita and proved its usefulness by applying it to nematicide screening.

Adsorption behavior and mechanism analysis of siloxane thickener for CO2 fracturing fluid on shallow shale soil

Reducing the adsorption of CO2 thickeners in soil particles of shallow shale is a pathway to protect the shale environment and water cycle. In this work, we first synthesized a siloxane-based thickener named AOB based on ring-opening polymerization and hydrosilylation reaction and discussed the influence of geological factors (including contents, pressure, temperature and shear rate) on the adsorption capacity of this thickener. The soil surface adsorption behavior of AOB was characterized by a self-assembled high-pressure soil adsorption device and the adsorption capacity was analyzed by Attenuated Total Reflectance Infrared Spectroscopy (ATR-FTIR). The results showed that AOB molecule belongs to the monolayer adsorption type on the soil surface by measuring the contents. Moreover, an inverse relationship was showed between pressure and adsorption capacity for AOB, while the significantly proportional relationship was presented for temperature and shear rate, respectively. It is found that the combined force formed by soil repulsion (originated from aromatic groups in AOB) and grid drag weakens the adsorption of AOB on the soil surface, resulting in a much lower adsorption capacity than the commercial PDMS. This work not only paves a new avenue for synthesizing siloxane-based polymer as CO2 thickener, but also provides an efficient measure to reduce the adsorption of thickener on the shallow soil.

Competitive adsorption of lead and cadmium on soil aggregate at micro-interfaces: Multi-surface modeling and spectroscopic studies

Aggregates are the basic structural units of soils and play a crucial role in metal migration and transformation. Combined contamination of lead (Pb) and cadmium (Cd) is common in site soils, and the two metals may compete for the same adsorption sites and affect their environmental behavior. Herein, the adsorption behavior of Pb and Cd on aggregates of two soils and contributions of soil components in single and competitive systems were studied by combining cultivation experiments, batch adsorption, multi-surface models (MSMs), and spectroscopic techniques. The results demonstrated that < 2 µm size aggregate was the dominant sink for Pb and Cd competitive adsorption in both soils. Compared with Pb, the adsorption capacity and behavior of Cd were affected greatly under competition. MSMs prediction revealed that soil organic matter (SOM) contributed the most to Cd and Pb adsorption on aggregates (> 68.4%), but the dominant competitive effect occurred on different sites for Cd adsorption (primarily on SOM) and Pb adsorption (primarily on clay minerals). Further, 2 mM Pb coexistence caused 5.9 − 9.8% of soil Cd conversion to unstable species (Cd(OH)2). Thus, the competitive effect of Pb on Cd adsorption cannot be ignored in soils with high content of SOM and fine aggregates.

Adsorption behaviour and mechanism of Avermectin-based pesticides in soil driven by H-bonds formed between the compounds and humus

Adsorption is an essential process in regulating pesticide distribution in soil. However, the relationship between soil adsorption mechanisms and the structure of the compounds is unclear. Different soil samples (S1, S2, S3) were selected for adsorption tests of avermectin B2a (AVB2a), avermectin B1a (AVB1a), and emamectin benzoate (EB) to investigate the influence of adsorbent properties and compound structure on the adsorption. The adsorption of the three chemicals by the three soils was extensive and irreversible. Soil adsorption of AVB2a was strongest, followed by AVB1a and EB. Soil adsorption tests of three avermectin-based pesticides showed that the –OH groups of the compounds were the main adsorption site. The humus fraction, a significant component of the soil organic matter, was extracted for further adsorption tests on the compounds. The humus fraction most strongly adsorbs AVB2a, followed by AVB1a and EB, indicating that –OH is also the main site for the humus fraction to adsorb compounds. Differences in the content of polar groups in the humus fraction by elemental analysis and 13C NMR analysis suggested that the –OH of avermectin-based pesticides interacts with the polar groups of humus. Quantum chemical calculations confirm that the –OH groups of the compound form hydrogen bonds with amino, hydroxyl, carboxyl, and carbonyl groups in the humus, thus demonstrating that hydrogen bonding is the mechanism by which avermectin-based pesticides are adsorbed to the soil.

Novel insights into the predominant factors affecting the bioavailability of polycyclic aromatic hydrocarbons in industrial contaminated areas using PLS-developed model

Bioavailability is recognized as a useful technical standard for risk assessment and pollution rehabilitation. However, knowledge on the bioavailability of polycyclic aromatic hydrocarbons (PAHs) in contaminated site soils is still limited, especially concerning the influential mechanism. With an abundance of soil collections from nine industrial areas in China, the bioavailabilities, as conceptually defined as bioconcentration factors (BCFs) of PAHs were analyzed using biomimetic extraction of hydroxypropyl-β-cyclodextrin (HPCD). Apart from the total content of PAHs varying with the different pyrogenic sources, the BCFs were greatly dependent on the soil physicochemical properties from the spatial scale and inversely proportional to the number of rings. Pearson correlation analysis indicated a weak relationship between bioavailability and the soil dissolved organic matter (DOM), pH and particle size. To incorporate the soil physicochemical properties and structural characteristics of PAHs determined by density functional theory (DFT), the optimum model for bioavailability was developed for BCFs by partial least square (PLS) analysis. The PLS-derived model was shown to be predictive within the applicability domain (AD). The structural characteristics, e.g., molecular polarizability and frontier orbital energy level that favor the soil adsorption of PAH isomers via dispersion interactions, and electron exchanges were indicated to be more impactful on bioavailability than soil environmental factors. However, soil factors should not be neglected, because the pH, DOM, etc. were significantly influential. It makes sense that the higher DOM causes greater bioavailability via increasing the free-dissolved fractions of PAHs. Interestingly, the effect of pH on bioavailability was spectrally validated by excitation-emission matrix (EEM) fluorescence, showing that the interaction between DOM and pyrene strengthened the fluorescence quenching of chromophores with the decline in pH.

Biochar affects the distribution of phosphorus in the soil profile by changing the capacity of soil to adsorb phosphorus and its distribution within soil macroaggregates

AbstractA large number of studies suggested that biochar can affect the adsorption of phosphorus (P) in soil and the distribution of P in soil aggregates, but it is still unclear whether biochar can indirectly affect the distribution of P in soil profiles through the influence of these two factors. Therefore, the ability of maize straw biochar to mitigate the risk of loss of P and improve the distribution of P in soil profile was evaluated by studying the capacity of soil to adsorb P from the soil and distribution fractions of it in diverse grain‐sized aggregates of soil. A 5‐year field trial site had received NPK fertilizer inputs at the same rate and one‐time biochar inputs at different rates (0, 15.75, 31.5 and 47.25 t ha−1) at the beginning of the experiment. Langmuir and Freundlich equations were fitted to the sorption data of the soil and suggested that the applicataion of biochar can promote P absorption capacity of soil, particularly when the rates of biochar applied were 31.5 and 47.25 t ha−1. Biochar can improve the stability of soil aggregates, and aggregates with particle sizes of 0.25–2 mm have the highest relative rate of contribution to various fractions of P in the soil. Biochar can promote the accumulation of most of the P fractions in soil aggregates, particularly when the rates of biochar applied were 31.5 and 47.25 t ha−1. The TP, Ca2‐P, Al‐P, Ca8‐P, moderately labile organic phosphorus (MLOP) and moderately resistant organic phosphorus (MROP) were primarily distributed in the 0–20 cm soil layer. Biochar increased the contents of MLOP and highly resistant organic phosphorus in the soil layer of 0–20 cm while reducing the contents of Al‐P, Fe‐P and occluded phosphorus (O‐P) in the soil layer of 0–20 cm and decreased the contents of TP, Ca8‐P, O‐P and MROP under the 20 cm soil layers. Biochar can change the distribution of P fractions along the soil profile by improving the adsorption of soil P and its distribution in the soil aggregates (0–20 cm).

Molecular Insights on the Adsorption of Polycyclic Aromatic Hydrocarbons on Soil Clay Minerals

The role of soil clay minerals is often ignored in studying the polycyclic aromatic hydrocarbons (PAHs) adsorption in soil. In this work, Monte Carlo and molecular dynamics simulations are used to study the adsorption of PAHs on several typical soil clay mineral surfaces, such as (0 0 1) and (0 0 − 1) of kaolinite, pyrophyllite, mica, and montmorillonite. Results show that van der Waals and cation-π interactions dominate the interactions between PAHs and mineral surfaces. Bridging oxygen atom on the mineral surface structure is the main site of hydrophobic interaction with PAHs. The cation-π effect can significantly enhance the PAHs adsorption energy. In addition, the adsorption on clay minerals is enhanced with the increase in the number of PAHs aromatic rings. In the presence of water, different minerals exhibit different adsorption properties. A layer of water molecules is formed close to the kaolinite (0 0 1) surface, reducing PAHs adsorption. The self-diffusion coefficient (D) of PAHs in Ca2+-montmorillonite is more significant than that in K+-mica, due to the different hydration properties of mineral cations. These findings reveal the adsorption mechanism of PAHs on the surfaces of typical clay minerals at a molecular scale and provide a new perspective for soil adsorption of PAHs.

Vulnerability assessment of soil cadmium with adsorption–desorption coupling model

The transfer rate and extent of cadmium (Cd) to groundwater and plants were controlled by soil adsorption and desorption processes, which directly affect agro-environment safety. Cd mobility shift is critical for the correct assessment of pollution vulnerability in different soil types. But the existing experimental methods and traditional quantitative prediction models are time-consuming and have a small scope of adaptation. In this paper, a coupling model of soil Cd adsorption and desorption was developed based on quantitative retention factors to explore the sensitivity of soil environmental factors for Cd pollution and quantify the spatial distribution pattern of Cd pollution vulnerability in highly heterogeneous soils in China. Results showed that: (1) The type of background electrolyte affected Cd adsorption and desorption capacity in soil solution. The performances of the single background electrolyte models (NaNO3, CaCl2, NaCl, KNO3) are better than the mixed background electrolyte model. (2) The contribution values of adsorption and desorption models were compared by sensitivity analysis. (3) Vulnerable areas of soil Cd pollution with higher risk than expected were selected by using the adsorption–desorption coupling model in China, including Tibet Autonomous Region, Xinjiang Uygur Autonomous Region, northwestern Sichuan Province, southeastern Qinghai Province, Guangdong Province, Guangxi Province, and Yunnan Province. Our adsorption–desorption coupling model can be used as an assessment tool expressing the soil Cd pollution vulnerability based on the available state of Cd, which is expected to provide the quantitative scientific basis for the formulation of Cd quality standard systems and soil remediation measures for regional soil environment protection.

Charcoal and Sago Bark Ash Regulates Ammonium Adsorption and Desorption in an Acid Soil

Excessive N fertilizer use in agriculture results in the release of inorganic N contaminants into surface and groundwater bodies, and other negative environmental effects. The combined application of N fertilizers with charcoal and sago bark ash could help reduce these negative impacts. The objective of this sorption study was to examine the effects of the co-application of charcoal and sago bark ash with ammonium chloride in regulating the adsorption and release of NH4+ in an acid soil. This soil used in the laboratory study was Bekenu series (Typic Paleudults). The treatments evaluated were: (i) 300 g soil only, (ii) 300 g charcoal only, (iii) 300 g sago bark ash only, (iv) 300 g soil + 15.42 g charcoal, (v) 300 g soil + 7.71 g sago bark ash, and (vi) 300 g soil + 15.42 g charcoal + 7.71 g sago bark ash. Regardless of the concentration of the isonormal solution, sago bark ash (T3) showed the highest NH4+ adsorption at equilibrium (Qe) and NH4+ desorbed (Qde). The results for T3 for Qe and Qde were 3.88 mg L−1 and 3.80 mg g−1, respectively, for the 400 mg N L−1 isonormal solution followed by T2 with values of 3.46 mg L−1 and 3.30 mg g−1, respectively. For treatments T2 and T3 that resulted in higher Qe and Qde for NH4+, soil was not included. However, in practical terms, any of the treatments T4, T5 and T6 that included mixing the amendments with soil are better since the results of these treatments were not significantly different in terms of Qe and Qde for NH4+. This is despite the fact that T4, T5 and T6 resulted in lower Qe and Qde for NH4+ compared to T2 and T3. The results also showed a positive linear relationship between NH4+ adsorption and the addition of N. This indicates that NH4+ can be retained temporarily by the amendments. The insignificant R2 (ranging from 0.10 to 0.38) of the Langmuir regression equations suggest that the NH4+ adsorption data did not fit the Langmuir isotherms well. Future studies could explore fitting the NH4+ sorption data into other sorption models. The higher adsorption of NH4+ by the treatment with charcoal is related to its high number of adsorption sites or negative charges of these materials. Incorporating charcoal and sago bark ash as soil amendments in agriculture has the potential to reduce the usage of chemical fertilizers. The reliance on commercial lime could also be reduced due to the alkaline characteristics of these materials. Therefore, the co-application of charcoal and sago bark ash could contribute to improve the utilization of N fertilizer by effectively controlling NH4+ availability for timely crop use, reducing losses, and preventing soil and water pollution.