pH-dependent Cation Exchange Capacity Research Articles

Soil electrical conductivity as a proxy for enhanced weathering in soils

To effectively monitor and verify carbon dioxide removal through enhanced weathering (EW), this study investigates the use of soil electrical conductivity (EC) and volumetric water content (θ) as proxies for alkalinity and dissolved inorganic carbon (DIC) in soil water. EC-θ sensors offer a cost-effective and straightforward alternative to traditional soil and water sampling methods. In a lab experiment, three different substrates were treated with NaHCO3 solutions to increase the alkalinity of the soil water and analyze the response. The combination of EC and θ to track the increase in carbonate alkalinity due to EW, and therefore CO2 consumption, is applicable for low cation exchange capacity (CEC) soil-substrates like the used quartz sand. However, the presence of organic material and pH-dependent CEC complicates the detection of clear weathering signals in soils. In organic-rich and clay-rich soils, only a high alkalinity addition has created a clear EC signal that could be distinguished from a non-alkaline baseline with purified water. Cation exchange experiments revealed that the used soil buffered alkalinity input and thereby might consume freshly generated alkalinity, initially mitigating CO2 uptake effects from EW application. Effective CEC changes with pH and pH buffering capacity by other pathways need to be considered when quantifying the CO2 sequestration potential by EW in soils. This should be estimated before the application of EW and should be part of the monitoring reporting and verification (MRV) strategy. Once the soil-effective CEC is raised, the weathering process might work differently in the long term.

A comparison of cation exchange capacity of organic soils determined by ammonium acetate solutions buffered at some pHs ranging between around field pH and 7.0

Cation exchange capacity (CEC) and exchangeable cations are among two of the most important measures for determining soil characteristics and soil quality. There are various methods of measuring CEC, including the ammonium acetate method of Schollenberger and Dreibelbis (1930) which is buffered at pH 7.0 and the barium chloride-triethanolamine method of Mehlich (1938) which is buffered at pH 8.2. These CEC measurements can result in values very different from the CEC of the soil at its field pH (effective CEC or CECe), especially in acidic soils, such as organic soils, with pH-dependent CEC. Therefore, the focus of this article is the comparison of CEC of organic soils obtained using the different pH level of the extraction solution. Five organic soil and two mineral soil samples were used in this experiment. The variables tested were CEC and exchangeable cations using ammonium acetate solutions buffered at several pHs; field pHs (acidic) of 3.8, 4.2, 4.8 and neutral (7.0). Each soil sample and pH level were replicated three times. The decomposition rate of the five organic soil samples was determined using the pyrophosphate color method, which proved to be sapric. The results indicated a positive relationship between pH and CEC value, the higher the pH buffer the much higher the CEC measured. There was a considerable difference of CEC values produced from the varying pHs of extraction solution. However, the pH of the extraction had no substantial effect on measured exchangeable cations.

Impacts of Cement Dust Emissions on Soils within 10km Radius in Ashaka Area, Gombe State, Nigeria

This study examines the impacts of cement dust emissions on physicochemical properties of soils within 10km radius from the plant (factory) in Ashaka area, Gombe State. Standard procedures used in soil sampling involves collection of soil samples along a transect aligned in the north-east to south-east direction in order to reflect the dominant two opposing air masses direction; the north-easterly (November-April) and south-westerly (May-October) air masses. Physicochemical analysis of the soils was carried out using standard laboratory procedures in the laboratory. Results of the analysis has revealed that the cement dust which contains high calcium has impacted the soils by increased soil pH, calcium (Ca) content, total bases, base saturation and pH dependent cation exchange capacity (CEC). The impact is observed most within the radius of 0-5km from the plant with a diminishing impact from the 5km towards the 10km radius. Similarly, results of the analysis has shown also that there are signs of slight impaction of bicarbonate (HCO3) and electrical conductivity (EC) arising from the cement dust on the soils. Recommendations were offered to monitor the dust falling on the soils through trapping and utilizing the dust emissions to cement.

Geochemical Modeling of Reactions and Partitioning of Trace Metals and Radionuclides during Titration of Contaminated Acidic Sediments

Many geochemical reactions that control aqueous metal concentrations are directly affected by solution pH. However, changes in solution pH are strongly buffered by various aqueous phase and solid phase precipitation/dissolution and adsorption/desorption reactions. The ability to predict acid-base behavior of the soil-solution system is thus critical to predict metal transport under variable pH conditions. This studywas undertaken to develop a practical generic geochemical modeling approach to predict aqueous and solid phase concentrations of metals and anions during conditions of acid or base additions. The method of Spalding and Spalding was utilized to model soil buffer capacity and pH-dependent cation exchange capacity by treating aquifer solids as a polyprotic acid. To simulate the dynamic and pH-dependent anion exchange capacity, the aquifer solids were simultaneously treated as a polyprotic base controlled by mineral precipitation/ dissolution reactions. An equilibrium reaction model that describes aqueous complexation, precipitation, sorption and soil buffering with pH-dependent ion exchange was developed using HydroGeoChem v5.0 (HGC5). Comparison of model results with experimental titration data of pH, Al, Ca, Mg, Sr, Mn, Ni, Co, and SO4(2-) for contaminated sediments indicated close agreement suggesting that the model could potentially be used to predictthe acid-base behavior of the sediment-solution system under variable pH conditions.

Using pH-dependent CEC to determine lime requirement

Controlling soil pH is important to ensure good crop yield. This study was conducted to determine whether the accuracy of the existing Shoemaker-McLean-Pratt (SMP) pH-buffer method could be improved by using the pH-dependent cation exchange capacity curve (CECpd). Soil pH, SMP and CECpd measurements were performed on 18 acid surface horizon soil samples, with textures from sandy loam to clay loam. These soils were incubated with three levels of calcium carbonate for 12 wk, after which the soil pH and the effective cation exchange capacity (CECe) were measured. The correlation coefficient (R2) for the CECpd and CECe curves was 0.96. The main factor affecting the slope of the curves is the soil organic matter content. The increase of CECe in the soil was also found to be directly proportional to the amount of lime applied, regardless of the type of soil. By using the slope of the Qv versus pH curve for each soil and the relationship between CECe and lime application, we were able to determine the lime required to raise the soil pH in water to 6.5. As an alternative to the current practice of using the SMP buffer, we propose that it should be possible to estimate the pH-dependent CEC curve from measurable soil properties (e.g., organic matter) and to estimate the lime requirement as the difference in CECpd between the existing and desired pH values. Once the slope of the Qv/pH relationship has been determined or estimated for a soil, the only measurement necessary for calculating lime requirement in subsequent years would be the soil pH. The proposed method would provide lime requirement estimates while decreasing the annual cost of soil analysis. Key words: Lime requirement, cation exchange capacity, Non-Ideal Competitive Adsorption, soil properties, organic matter, Fe oxides

Sorption of Three Tetracyclines by Several Soils: Assessing the Role of pH and Cation Exchange

Tetracyclines (TCs) are widely used in veterinary medicine for treatment and prevention of disease and are present in animal waste products. Detection of TCs in soil, sediments, and water, and the growing concern of their potentially adverse effect on natural ecosystems have resulted in a need to understand their behavior in aqueous soil systems. TCs have multiple ionizable functional groups such that at environmentally relevant pH values, they may exist as a cation (+ 0 0), zwitterion (+ - 0), or a net negatively charged ion (+ - -), which complicates predicting their sorption, availability, and transport. We investigated the sorption of oxytetracycline (OTC), tetracycline (TC), and chlortetracycline (CTC) by several soils varying in pH, clay amount and type, cation exchange capacity (CEC), anion exchange capacity (AEC), and soil organic carbon in 0.01 N CaCl2, 0.001 N CaCl2, and 0.01 N KCI. All three TCs are highly sorbed, especially in acidic and high clay soils. When normalized to CEC, sorption tends to decrease with increasing pH. A sorption model in which species-specific sorption coefficients normalized to pH-dependent CEC (Kd+00, kd+-0, and kd+--) and weighted by the pH-dependent fraction of each species fit the data well across all soils except for a soil rich in gibbsite and high in AEC. Resulting kd+00 values were more than an order of magnitude larger than values for either kd+0 and kd+--values such that kd+00 alone described most of the sorption observed as a function of pH for eight soils that varied in their mineralogy and pH (pH ranged from 4 to 8).

A New Method for the Simultaneous Measurement of pH‐Dependent Cation Exchange Capacity and pH Buffering Capacity

The pH dependent cation exchange capacity (CEC) and pH buffering capacity (pH BC) are two crucial properties in soil fertility management of variable charge soils. However, they are rarely measured, since most of the existing methods are cumbersome and time‐consuming. We propose a new method, based on the silver‐thiourea (AgTU) method, for the simultaneous measurement of pH dependent CEC and the pH BC. In general, this method consists of first increasing the pH of the soil to about pH 7 and then gradually acidifying the soil by titrating with HNO3, while measuring pH and CEC after each equilibration. This way, both CEC at different pH values and the pH BC are measured on the same sample, without the multiple washing steps needed in other procedures. Various aspects of the proposed method were tested and the results obtained were compared with the ion adsorption method. The modified AgTU method obtains field relevant results and is suitable for the routine analysis of large numbers of samples.

Chemical equilibria model of strontium-90 adsorption and transport in soil in response to dynamic alkaline conditions.

Strontium-90 is a major hazardous contaminant of radioactive wastewater and its processing sludges at many Department of Energy (DOE) facilities. In the past, such contaminated wastewater and sludge have been disposed in soil seepage pits, lagoons, or cribs often under highly perturbed alkaline conditions (pH > 12) where 90Sr solubility is low and its adsorption to surrounding soil is high. As natural weathering returns these soils to near-neutral or slightly acidic conditions, the adsorbed and precipitated calcium and magnesium phases, in which 90Sr is carried, change significantly in both nature and amounts. No comprehensive computational method has been formulated previously to quantitatively simulate the dynamics of 90Sr in the soil-groundwater environment under such dynamic and wide-ranging conditions. A computational code, the Hydrologic Utility Model for Demonstrating Integrated Nuclear Geochemical Environmental Responses (HUMDINGER), was composed to describe the changing equilibria of 90Sr in soil based on its causative chemical reactions including soil buffering, pH-dependent cation-exchange capacity, cation selectivity, and the precipitation/dissolution of calcium carbonate, calcium hydroxide, and magnesium hydroxide in response to leaching groundwater characteristics including pH, acid-neutralizing capacity, dissolved cations, and inorganic carbonate species. The code includes a simulation of one-dimensional transport of 90Sr through a soil column as a series of soil mixing cells where the equilibrium soluble output from one cell is applied to the next cell. Unamended soil leaching and highly alkaline soil treatments, including potassium hydroxide, sodium silicate, and sodium aluminate, were simulated and compared with experimental findings using large (10 kg) soil columns that were leached with 90Sr-contaminated groundwater after treatment. HUMDINGER's simulations were in good agreement with dynamic experimental observations of soil exchange capacity, exchangeable cations, total 90Sr, and pH values of layers within the soil columns and of column effluents.

Clay minerals in basin of Mexico lacustrine sediments and their influence on ion mobility in groundwater

Semiconfined aquifers used as the principal water supply within the Basin of Mexico are overlain by lacustrine deposits that provide a degree of protection from contamination associated with metropolitan Mexico City. Solute transport behavior and the nature of chemical interactions with mineral components in these sediments is poorly understood. The objectives of this paper were to identify the clay mineral phases of the lacustrine sediments and to determine the significance of the exchange properties of the day minerals on contaminant transport processes. Samples obtained from two cores were separated into sand, silt, and clay-size fractions. The clay-size fraction was analyzed by X-ray diffraction and Fourier transform infrared spectroscopy and for specific surface area and pH-dependent cation and anion exchange capacity. The clay-sized fraction averaged 56% of the sediment. Analyses indicated that the clay was predominantly composed of a Si-rich allophane with approximately 30% montmorillonite. Halloysite was also present to a depth of about 1.5 m, but was absent deeper in the sediment. Kaolinite and illite, reported in previous studies, and imogolite were not found in the samples. Solute transport in the sediment was modeled to demonstrate the impact of exchange properties imparted by the allophane compared to other possible clay mineral assemblages. The predominance of allophane in the Basin of Mexico sediments is responsible for many of the fundamental characteristics of the material including: high porosity (0.8–0.9), high water content (200–400%), and an extremely high and pH-dependent cation exchange capacity. The pH of the pore water within the lacustrine sediments of the Basin of Mexico is typically between 6.5 and 12. Measured cation exchange values ranged from ≈ 450 meq kg −1 at pH 6.5 to ≈ 650 meq kg −1 at pH 12 which could produce variable cation mobility in the semiconfining aquitard. The simulations illustrated that allophane is very effective for retarding the leaching of cationic metals compared to other possible clay mineral suites, particularly at high pH values. Simulation of long-term infiltration indicated that trace metals will become concentrated in the clay matrix adjacent to fracture faces in the sediment near ground surface. As such, the aquitard behaves as an efficient barrier to the infiltration of cationic metals into the underlying aquifer.

X-ray diffraction characteristics and related properties of Smectites in some Canadian soils

The X-ray diffraction (XRD) peak intensities of smectites in Chernozemic and related soils of Western Canada are generally low, in contrast to the high peak intensities of smectites in Podzolic soils of Eastern Canada and those of standard smectite samples. Consequently, X-ray quantitative analysis based on standard smectite samples may underestimate the amount of smectite and overestimate the amount of noncrystalline material in western Canadian soils. This study was undertaken to find the reasons for the weak XRD peak intensities of western soil smectites in terms of their purity and crystallinity. The Tiron dissolution method extracted only small amounts of noncrystalline material from the western soil clays and had little effect on XRD characteristics. The cation exchange capacity (CEC) hysteresis (or pH dependent CEC) between pH 3.5 and 11.0 of the western soil clays was also relatively small which confirmed the absence of significant amounts of noncrystalline material in these soil clays. Observed deviations of XRD positions from true basal spacings indicated that the western soil smectite particles were consistently thinner than the eastern soil smectite and Wyoming montmorillonite particles. Electron microscope observations supported these results. Because the diffraction intensity is proportional to the square of the particle thickness, the thinness of the western soil smectite particles appears to be a major factor in reducing their peak intensities. Thus, to obtain comparable peak intensity data from different soil smectites, particle thickness should be taken into account. Since it is often not practical to measure particle thickness, a procedure for quantifying smectite in soils that avoids this measurement is proposed. Key words: Peak intensity, crystallinity, particle size, particle thickness, Chernozemic smectite, Podzolic smectite

Cation Exchange Properties of Pine Bark Growing Media as Influenced by pH, Particle Size, and Cation Species

Abstract Pine bark cation exchange capacity (CEC) (by Ba/Mg exchange on four particle size fractions) increased regularly from 38 to 98 meq/100 g between pH 4 and 7. Decreasing particle size from <2.38 to <0.05 mm did not result in the expected large increases in bark CEC. The Ba/Mg CEC of unsieved bark samples was less than that determined by the weighted average of component size fractions. Monovalent/monovalent-determined CEC was higher than Ba/Mg, indicating that a number of differing charge-specific sites are involved. The pH-dependent CEC increase between pH 4 and 7 was greater for divalent exchange than for monovalent. Ammonium/K CEC was higher than K/NH4 CEC, probably due to enhanced NH4 adsorption by carboxyl groups. Infrared analysis of pine bark revealed that surface functional group composition is similar to soil organic matter. The accurate measurement of CEC in pine bark is complicated by solution pH and ionic strength, as well as by the cations employed for exchange.

Cation exchange capacity measurements

Abstract Soil cation exchange capacity (CEC) measurements are important criteria for soil fertility management, vaste disposal on soils, and soil taxonomy. The objective of this research was to compare CEC values for arable Ultisols from the humid region of the United States as determined by procedures varying widely in their chemical conditions during measurement. Exchangeable cation quantities determined in the course of two of the CEC procedures were also evaluated. The six procedures evaluated were: (1) summation of N NH4OAc (pH 7.0) exchangeable Ca, Mg, K, and Na plus BaCl2 ‐ TEA (pH 8.0) exchangeable acidity; (2) N Ca(OAc)2 (pH 7.0) saturation with Mg(OAc)2 (pH 7.0) displacement of Ca2+; (3) N NH4OAc (pH 7.0) saturation with NaCl displacement of NH4 +; (4) N MgCl2 saturation with N KCl displacement of Mg2+; (5) compulsive exchange of Mg2+ for Ba2+; and (6) summation of N NH4OAc (pH 7.0) exchangeable Ca, Mg, K, and Na plus N KCl exchangeable AJ. The unbuffered procedures reflect the pH dependent CEC ...

EFFECT OF LIMING ACID SOILS ON POTASSIUM AVAILABILITY

Calculations performed using a Gapon-type exchange equation agreed with experimental observations of the influence of lime on solution potassium levels in a soil with a high pH-dependent cation exchange capacity (CEC). As lime additions increase the pH and the CEC, solution potassium decreases. Amounts of potassium that must be added to the soil to reach solution pK-1/2p(Ca + Mg) = 2 were calculated to increase from 84 parts per 2 million at no lime to 1150 parts per 2 million for the high-lime level. The influence of lime on potassium availability could be even more severe if not for the decrease in potassium selectivity relative to calcium with increasing charge density.

ETUDE SUR LA VALEUR DES CRITERES CHIMIQUES PROPOSES PAR LA COMMISSION PEDOLOGIQUE DU CANADA POUR LA CLASSIFICATION TAXONOMIQUE DES SOLS DU QUEBEC

Correlations were made among chemical criteria used for taxonomic soil classificaton. The compared tests were: oxalate Δ (Fe + Al), pyrophosphate-extractable (Fe + Al), oxalate-extractable Al, pH-dependent cation exchange capacity (ΔCEC), ratios of pyrophosphate-extractable (Fe + Al) over clay or over dithionite-extractable (Fe + Al), and finally soil pH measured in 1 M NaF. Significant correlations were found among various measured parameters. However, no single test was found to be reliable as a single criterion when applied to the taxonomic classification of Quebec soils. The two chemical tests, pyrophosphate-extractable (Fe + Al) and its ratio over clay, combined with morphologic criteria appeared useful for classifying Quebec Podzols. A few soils, which presented discrepancies from chemical criteria were found difficult to classify, thus suggesting the possibility of establishing new sub-groups in the Canadain soil taxonomic classification system.

Charge Characteristics and Nitrate Retention By Two Andepts from South‐Central Chile

AbstractCharge characteristics of two Andepts, or volcanic‐ash‐derived soils, from south‐central Chile were studied using a combination of nitrate retention, pH dependent cation exchange capacity and pH titration curves. Nitrate retention was determined by washing the soil with KNO3 solutions having the appropriate concentration and pH and determining the amount of nitrate adsorbed. Net surface charge was determined from potentiometric titration curves conducted using a batch technique.Results show that both soils have a complex mixture of pH dependent and permanent charge surfaces. The maximum pH dependent charge is in the range of 15 to 20 meq/100g at either very low or very high pH and high electrolyte concentration. The permanent negative charge is in the range of 45 meq/100g and is apparently the result of ion substitutions and lattice vacancies in X‐ray amorphous oxides and hydrous oxides rather than 2:1 type clay minerals.

Interactions of pH‐Dependent and Permanent Charges of Clays: III. Calcium and Rb Activities versus Soybean Root Uptake from Bentonite and Illite Suspensions

Panther Creek bentonite and Fithian illite suspensions were reciprocally saturated with Ca and Rb. 45Ca and 86Rb were added as tracers for Ca and Rb, respectively. Increments of H-clay were added to aliquots of the Ca-Rb saturated clays to progressively decrease the base saturation of the clays. After equilibration, the activities of Ca and Rb were measured in the clay suspensions with the Orion divalent and the Beckman monovalent electrodes. Uptake of Ca and Rb by excised soybean roots was determined from the radioactivity of each tracer. Decreased Ca saturation (increased Rb) shifted Ca ions from pH-dependent to permanent charges causing increased bonding and thus decreased Ca activities, fractions active, and root uptake. Except where specific ion bonding effects interfered, increased Rb saturation increased Rb activities and uptake—even though Rb fractions active were decreased, due to greater permanent charge bonding. With the same exception, additions of H-clays equilvalent to pH-dependent cation exchange capacity minimized Ca and Rb uptake as well as their activities and fractions active. Thus activity data generally correlated closely with uptake data, except where there were good reasons for this not to occur.

ESTIMATION OF THE INORGANIC AND ORGANIC pH-DEPENDENT CATION EXCHANGE CAPACITY OF THE B HORIZONS OF PODZOLIC AND BRUNISOLIC SOILS

Heating in hydrogen peroxide, dilute oxalic acid, and dilute aluminum oxalate did not change the effective cation exchange capacity (CEC) or the pH-7 CEC of Wyoming bentonite and Alberni clay soil containing excess Al(OH)x. This indicated that treatment of soils with H2O2 to oxidize organic matter and the possible production of oxalates during oxidation did not change the CEC values of the inorganic fraction of soils even if some clay exchange sites were blocked by hydrous oxides of Al.With soils of pH less than approximately 5.4, oxidation of organic matter did not change the effective CECs although the pH-7 CEC values were decreased. Thus, organic matter in acid soils appeared to have little or no effective CEC. Because of this and the negligible effect of H2O2 oxidation on the CEC values of clays, the difference of the pH-7 CEC of soils before and after H2O2 oxidation provided a simple means of estimating the amount of organic pH-dependent CEC in acid soils.The amount of organically derived pH-dependent CEC was determined in a number of soils by means of peroxide oxidation. The technique provided a useful indication of the quantities of sesquioxide–organic matter complexes accumulated in medium- and fine-textured soils.