Particle concentration effect and adsorption reversibility

Leaching behaviour of 226Ra from uranium tailings and adsorption behaviour in geotechnical medias

The leaching process of uranium tailings is a typical water-rock interaction. The release of 226Ra from uranium tailings depends on the nuclides outside the intrinsic properties of uranium tailings on the one hand, and is influenced by the water medium on the other. In this paper, a uranium tailings repository in southern China was used as a research object, and uranium tailings at different depths were collected by drilling samples and mixed to analyze the 226Ra occurrence states. Static dissolution leaching experiments of 226Ra under different pH conditions, solid-liquid ratio conditions, and ionic strength conditions were carried out, and the adsorption and desorption behaviours of 226Ra in five representative stratigraphic media were investigated. The results show that 226Ra has a strong adsorption capacity in representative strata, with adsorption distribution coefficient Kd values ranging from 1.07E+02 to 1.29E+03 (mL/g) and desorption distribution coefficients ranging from 4.97E+02 to 2.71E+03 (mL/g), but the adsorption is reversible. The 226Ra in uranium tailings exists mainly in the residual and water-soluble states, and the release of 226Ra from uranium tailings under different conditions is mainly from the water-soluble and exchangeable state fractions. Low pH conditions, low solid-liquid ratio conditions and high ionic strength conditions are favourable to the release of 226Ra from uranium tailings, so the release of 226Ra from uranium tailings can be reduced by means of adjusting the pH in the tailings and setting up a water barrier. The results of this research have important guiding significance for the management of existing uranium tailings ponds and the control of 226Ra migration in groundwater, which is conducive to guaranteeing the long-term safety, stability and sustainability of uranium mining sites.

Research on leaching behavior of uranium from a uranium tailing and its adsorption behavior in geotechnical media

The release of uranium from uranium tailings into the aqueous environment is a complex process controlled by a series of interacting geochemical reactions. In this paper, uranium tailings from a uranium tailings pond in southern China were collected at different depths by means of borehole sampling and mixed to analyze the fugacity state of U. Static leaching experiments of U at different pH, oxidant concentration and solid-to-liquid ratios and dynamic leaching experiments of U at different pH were carried out, and the adsorption and desorption behaviour of U in five representative stratigraphic media were investigated. The results show that U is mainly present in the residue state in uranium tailings, that U release is strong in the lower pH range, that the leached U is mainly in the form of U(VI), mainly from the water-soluble, Fe/Mn oxides and exchangeable fraction of uranium tailings, and that the reduction in U leaching at higher pH is mainly due to the combined effect of precipitation formation and larger particle size of platelets in uranium tailings. Experiments with different oxidant concentrations and solid-liquid ratios showed that the oxygen-enriched state and low solid-liquid ratios were favorable for the leaching of U from uranium tailings. Adsorption and desorption experiments show that U is weakly adsorbed in representative strata, reversibly adsorbed, and that U is highly migratory in groundwater. The present research results have important guiding significance for the management of existing uranium tailings ponds and the control of U migration in groundwater, which is conducive to ensuring the long-term safety, stability and sustainability of uranium mining sites.

Highly efficient adsorption of antibiotic ciprofloxacin hydrochloride from aqueous solution by diatomite-basic zinc chloride composites

The antibiotic ciprofloxacin (CIP) is used to treat a variety of bacterial infections, yet it poses significant health risks to aquatic environments. While adsorption is a promising technique for CIP removal, current adsorption capacities remain limited. In this study, we introduce a diatomite and basic zinc chloride composite (ZnHC-Dt) prepared using a straightforward deposition method, with the ability to achieve highly efficient ciprofloxacin removal. ZnHC-Dt is characterized using field emission scanning electron microscopy (SEM), X-ray diffraction analysis (XRD), Fourier transform infrared spectroscopy (FTIR), and the Brunauer-Emmett-Teller method (BET). We also assess the zeta potential. The optimized ZnHC-Dt adsorbent, achieved at a mass ratio of 0.45 with ZnHC/(ZnHC+Dt), is adopted with a CIP adsorption capacity of 831.96 mg/g at 25 °C, broad pH adaptability (within 3.0-10.0), rapid adsorption rate (reaching equilibrium in 4 h), and stable performance under Na+ ionic strength. The CIP adsorption process follows pseudo-second-order kinetics and aligns well with the Langmuir adsorption model. The high adsorption capacity of ZnHC-Dt can be attributed to electrostatic attraction, hydrogen bonding, surface complexation, and available adsorption sites. During the desorption process, the CIP removal rate retains 65.33% effectiveness after five cycles. The results suggest that ZnHC-Dt holds significant potential for CIP removal in aqueous solutions.

Solution to the particle concentration effect on determining Kd value of radionuclides

The particle concentration effect on Kd values of radionuclides has been observed but the underlying mechanism remains controversial. The hope is to use the relationship between particle concentration, adsorption-desorption isotherms and reversibility, in combination with surface component activity of model (SCA model), to solve this issue. 137Cs, 60Co, 90Sr were used as tracers, batch experiments were conducted in freshwater-sediment and seawater-sediment. The experiment of each radionuclide was designed with five different particle concentrations Cp, and for each Cp there were seven different initial concentrations C0. After adsorption experiments, four consecutive desorption experiments were carried out. At the fourth desorption experiment, radionuclide concentrations in the supernatant and sediment were measured. The results showed that adsorption and single desorption data of 137Cs, 60Co, 90Sr might be described by linear isotherms. 137Cs was reversible in the seawater-sediment, so hysteresis angles of the five-particle concentration were approximately 0°, all adsorption and desorption data could be classified into one line. In the remaining systems, besides the adsorption and single desorption isotherms moved upward with the decrease of particle concentration, hysteresis angles and irreversibility also increased, thus, the particle concentration effect was obvious. The reversible and resistant component concentrations calculated by adsorption, single desorption and consecutive desorption isotherm were linear functions of equilibrium concentration Ce1, respectively. Data from adsorption and desorption experiments with particle concentration effect could be classified into the same line using the Freundlich-SCA model. The results of this study indicate that the particle concentration effect is related to reversibility. When adsorption isotherm and single desorption isotherm are both linear, consecutive desorption isotherm, reversible and resistant component concentrations approach linearity too. After the Freundlich-SCA model eliminated the particle concentration effect on adsorption and desorption data, the data can be used to predict the adsorption, single desorption isotherm and Kd value at any particle concentration.

Isotherm models for adsorption of heavy metals from water - A review

Adsorption is a widely used technology for removing and separating heavy metal from water, attributed to its eco-friendly, cost-effective, and high efficiency. Adsorption isotherm modeling has been used for many years to predict the adsorption equilibrium mechanism, adsorption capacity, and the inherent characteristics of the adsorption process, all of which are substantial in evaluating the performance of adsorbents. This review summarizes the development history, fundamental characteristics, and mathematical derivations of various isotherm models, along with their applicable conditions and application scenarios in heavy metal adsorption. The latest progress in applying isotherm models with a one-parameter, two-parameter, and three-parameter in heavy metal adsorption using carbon-based materials, which has gained much attention in recent years as low-cost adsorbents, is critically reviewed and discussed. Several experimental factors affecting the adsorption equilibrium, such as solution pH, temperature, ionic strength, adsorbent dose, and initial heavy metal concentration, are briefly discussed. The criteria for selecting the optimum isotherm for heavy metal adsorption are proposed by comparing various adsorption models and analyzing mathematical error functions. Finally, the relative performance of different isotherm models for heavy metal adsorption is compared, and the future research gaps are identified.

Effect of natural geotextile on the cotransport of heavy metals (Cu2+, Pb2+, and Zn2+) and kaolinite particles

A cotransport study of heavy metals and kaolinite particles in sand column with and without flax geotextiles was carried out. The objectives were to evaluate the potential role of kaolinite in heavy metals transfer and to analyse the influence of flax geotextiles on the transfer of these pollutants. The adsorption rates of heavy metals on the kaolinite particles were, respectively, 53%, 65% and 25% for copper, lead, and zinc. The injection of kaolinite with heavy metals resulted in a significant decrease in the retention efficiency of copper and lead in the filter. The presence of kaolinite in the injected solution has virtually no influence on the effectiveness of zinc fixation in the filter. The retention of heavy metals is in the order of Zn > Cu > Pb with a significant drop of retention efficiency of 34% for copper, 67% for lead, and less than 1% for zinc. The presence of kaolinite in the injected solution reversed the retention order of heavy metals when metals solution was injected alone. Flax geotextiles increase the ability of the filter to retain soluble and attached heavy metals. It improves the sand retention capacity and it retains soluble and attached metals in its structure.

Examination of current adsorption models for Pb(II) and Cu(II) adsorption onto Fe3O4@Mg2Al-NO3 Layered Double Hydroxide in aqueous solution

The adsorption of Pb(II) and Cu(II) onto Fe3O4@Mg2Al-NO3 Layered Double Hydroxide (LDH) as a function of Fe3O4@Mg2Al-NO3 LDH concentration was studied. An adsorbent concentration effect ( Cs effect), namely adsorption isotherm declines as adsorbent concentration ( Cs) increases, was observed. The experimental data were fitted to the adsorption models including the classic Freundlich model, the metastable-equilibrium adsorption theory, the flocculation model, the power function model, and the surface component activity model. The results show that the Freundlich-type metastable-equilibrium adsorption equation, the power function model, and the Freundlich-surface component activity equation can adequately describe the Cs effect observed in the batch adsorption tests as all the correlation coefficients ( R2) of the nonlinear plots are higher than 0.96. In other words, their intrinsic parameters simulated from the experimental data are independent of Cs value. It is considered that the Freundlich-surface component activity equation is the best model to describe the Cs effect of the studied adsorption systems by Akaike Information Criterion evaluation criterion.

Adsorption performance of Cr(vi) onto Al-free and Al-substituted ferrihydrites

Al-Substituted ferrihydrite exhibits a higher adsorption capacity for Cr(vi) than pure ferrihydrite.

Chemical attenuation of arsenic by soils across two abandoned mine sites in Korea

The chemical attenuation of As by soils from abandoned mine sites was evaluated. Several soil samples, including As contaminated soil from the mine impacted areas, as well as As-free soils down-gradient from the mine sites, were collected across abandoned mine sites. Leaching and adsorption experiments were conducted under batch and 1-D water flow conditions. The cumulative As mass from 10 step sequential leaching experiments with six As contaminated soils, using 10 mM CaCl₂ solution, was less than 1% of the total As present in soils, indicating that As in contaminated soils is strongly adsorbed onto soil particles, which can serve as a long term potential As source. As adsorption by As-free soils was clearly nonlinear, with Freundlich N values (sorption nonlinearity) ranging from 0.56 to 0.87. Both the total As content in mine soils and the concentration-specific adsorption coefficient for arsine-free soils were best described by coupling the pH with various forms of Fe/Al oxides. In the breakthrough curves (BTCs) for As contaminated soils, an initial high concentration of As (called first-flush) was observed, and this flush export leveled off after the displacement of a few pore volumes. In the BTCs from layered soils, where clean down-gradient soils were overloaded above the mine soil, the appearance of measurable As was retarded, showing that the As attenuation by soils was effective in a flow water system. Also, the observed perturbation in the concentration of As during flow interruption supports that leaching/attenuation of As via flowing water occurs under nonequilibrium conditions. The results from both batch leaching/adsorption and column displacement experiments strongly suggested that the leaching of As from mine soils was rate limited and the risk of As leaching from soils can be mitigated by attenuation mechanisms, such as adsorption, provided by down-gradient clean soils.

Adsorption of copper and zinc on pseudomonas putida CZ1: Particle concentration effect and adsorption reversibility

The adsorption and desorption processes of Cu(II) and Zn(II) on the biomass of Pseudomonas putida CZ1 as a function of particle concentrations (C(p)) were studied. In a 0.01 M KNO3 solution, the Cu-biomass and Zn-biomass adsorption systems displayed a clear C(p) effect. The overall adsorption isotherms under three C(p) conditions could be described as a Freundlich-type C(p) effect isotherm equation: gamma = 2.553C(p)(-0.7106) C(eq)(0.8971) for Cu-biomass system, gamma = 2.412C(p)(-0.8305) C(eq)(0.6504) for Zn-biomass system. The results of experiments, designed to eliminate several typical sources of experimental artifact, agree with the prediction of the metastable-equilibrium adsorption theory. Results from laboratory equilibration studies also indicate that biomass-adsorbed Cu(II) or Zn(II) fractions may be comprised of both reversibly and strongly bound or resistant components. A computational method has been derived to allow prediction of the magnitude of the reversible and more strongly adsorbed Cu(II) or Zn(II) fractions from conventional isotherm data. This methodology provides an initial quantitative approximation of the strongly bound, resistant, biomass fractions while utilizing relatively simple experimental adsorption-desorption data.

Ion adsorption components in liquid/solid systems

Experiments on Zn2+ and Cd2+ adsorptions on vermiculite in aqueous solutions were conducted to investigate the widely observed adsorbent concentration effect on the traditionally defined adsorption isotherm in the adsorbate range 25-500 mg/L and adsorbent range 10-150 g/L. The results showed that the equilibrium ion adsorption density did not correspond to a unique equilibrium ion concentration in liquid phase. Three adsorbate/adsorbent ratios, the equilibrium adsorption density, the ratio of equilibrium adsorbate concentration in liquid phase to adsorbent concentration, and the ratio of initial adsorbate concentration to adsorbent concentration, were found to be related with unique values in the tested range. Based on the assumption that the equilibrium state of a liquid/solid adsorption system is determined by four mutually related components: adsorbate in liquid phase, adsorbate in solid phase, uncovered adsorption site and covered adsorption site, and that the equilibrium chemical potentials of these components should be equalized, a new model was presented for describing ion adsorption isotherm in liquid/solid systems. The proposed model fit well the experimental data obtained from the examined samples.

EXAFS studies on adsorption–desorption reversibility at manganese oxides–water interfaces

Microscopic structures of Zn(II) surface complexes adsorbed at the manganite (gamma-MnOOH)-water interface were studied using extended X-ray absorption fine structure (EXAFS) spectroscopy. Quantitative analysis of the first sphere showed that, in a 0.1 M NaNO(3) solution of pH 7.5, Zn(II) was adsorbed as a mixture of tetrahedral and octahedral structure (ZnO(4,6) polyhedra) and the average Zn-O distance was 2.00+/-0.01 A. EXAFS analysis of the second sphere showed that two typical atomic Zn-Mn distances of 3.07+/-0.01 and 3.52+/-0.02 A existed in the surface complexes, indicating that there were two types of linkage, i.e., the edge-linkage of high affinity and the corner-linkage of low affinity, between the ZnO(4,6) polyhedra and the MnO(6) octahedra of the manganite. Macroscopic adsorption-desorption experiments showed that adsorption of Zn(II) onto manganite was largely irreversible and the stronger edge-linkage mode was found to be responsible for the adsorption irreversibility. This result provided direct evidence from the molecular level for the basic hypothesis of the metastable-equilibrium adsorption (MEA) theory that adsorption density is not a thermodynamic state variable because a given value of adsorption density could have different values of chemical potential, depending on the proportion between the edge and corner linkage modes.

EXAFS studies on adsorption–desorption reversibility at manganese oxide–water interfaces

Microscopic structures of Zn(II) adsorbed at delta-MnO(2)-water interfaces were studied using extended X-ray absorption fine structure (EXAFS) spectroscopy. In a 0.1 M NaNO(3) solution of pH 5.50, hydrous Zn(II) was adsorbed onto the solid surface in the form of octahedral coordination. Adsorbed octahedral Zn(II) was located above and below the vacancy sites of delta-MnO(2). Each Zn was coordinated on one side to H(2)O molecules forming an H(2)O sheet and on the other side to oxygen atoms shared with layer MnO(6) octahedra forming a corner-sharing octahedral interlayer complex. The average Zn-O and Zn-Mn distances were 2.07+/-0.01 and 3.52+/-0.01 A, respectively. Macroscopic adsorption-desorption isotherms showed that, in contrast to that of the Zn-gamma-MnOOH system, adsorption of Zn(II) on delta-MnO(2) was highly reversible. EXAFS results indicated that the highly reversible adsorption was due to the weak adsorption mode of the corner-sharing linkage between the adsorbate and adsorbent polyhedra.

Adsorption-desorption of phosphate on airborne dust and riverborne particulates in East Mediterranean seawater

The potential importance of adsorption-desorption behavior of phosphorus (P) on the East Mediterranean (E. Med) P cycle was investigated. Contrasting adsorption behavior between Saharan dust (SD) and Nile particulate matter (Nile PM) was observed. SD was a source of P to the region, which released an average of 3.3 +/- 0.3 micromolP/g into the surface seawater and showed no adsorption ability under the conditions close to the E. Med deep water. Saharan dust is therefore unlikely to be the reason for P limitation in the region. By contrast, Nile PM acted dual roles of a sink and source of P in different waters (surface seawater, deep seawater, and river water). A new crossover-type adsorption-desorption model explained the contrasting adsorption behavior and the dual nature of natural particles. The model indicates that when natural particles are transported between different waters, they can be a sink (adsorption) or a source (desorption) of phosphorus depending on the "specific concentration (lambda)", which is the ratio between the aqueous P concentration and the zero equilibrium P concentration (EPC0). EPC0 refers to the solute concentration value where the adsorption isotherm crosses over the aqueous concentration axis. When lambda > 1, adsorption occurs, whereas when lambda < 1, desorption occurs. The model added a general development to the methodology of adsorption isotherm, where, for the first time, effects of solute concentration, solid concentration, and aqueous medium (EPC0) on the adsorption and desorption of P in natural waters were simultaneously described by a single equation. Using the model, it was quantitatively reconstructed that particles emitted during the pre-1964 Nile floods could be a major source of P to Egyptian coastal waters (up to 4800 tonsP/yr), greater than the dissolved P flux (approximately 3200 tonsP/yr), but a trapper of dissolved phosphate in E. Med deep waters.