Abstract: The sorption performance of a modified carbon black was explored with respect to arsenic removal following batch equilibrium technique. Modification was accomplished by refluxing the commercial carbon black with an acid mixture comprising HNO3 and H2SO4. Modification resulted in the substantial changes to the inherent properties like surface chemistry and morphology of the commercial carbon black to explore its potential as sorbent. The suspension pH as well as the point of zero charge (pHpzc) of the material was found to be highly acidic. The material showed excellent sorption performance for the removal of arsenic from a synthetic aqueous solution. It removed 93% arsenic from a 50 mg/L solution at equilibration time. The modified carbon black is capable of removing arsenic in a relatively broad pH range of 3–6, invariably in the acidic region. Both pseudo-first-order and second-order kinetics were applied to search for the best fitted kinetic model to the sorption results. The sorption process is best described by the pseudo-second-order kinetic. It has also been found that intra-particle diffusion is the rate-controlling step for the initial phases of the reaction. Modelling of the equilibrium data with Freundlich and Langmuir isotherms revealed that the correlation coefficient is more satisfactory with the Langmuir model although Freundlich model predicted a good sorption process. The sorption performance has been found to be strongly dependent on the solution pH with a maximum display at pH of 5.0. The temperature has a positive effect on sorption increasing the extent of removal with temperature up to the optimum temperature. The sorption process has been found to be spontaneous and endothermic in nature, and proceeds with the increase in randomness at the solid–solution interface. The spent sorbent was desorbed with various acidic and basic extracting solutions with KOH demonstrating the best result (85% desorption).

Abstract: A commercial grade carbon black was chemically modified using mineral acids (either with HNO3 or H2SO4 or mixture) and the sorption performance of the virgin and modified forms were investigated. Chemical modification resulted in the creation of surface acidic functional groups (COOH, SO2OH) and was verified by FTIR spectra. This was further verified by TGA analysis revealing higher weight loss characteristics of the modified carbons in comparison to virgin carbon black. Morphological changes were observed from BET surface area measurements and SEM analysis. XRD study revealed the change of graphitic crystallite size as a result of modification. The suspension pH of the materials in deionized water and the point of zero charge (pHpzc) in inert electrolyte were determined. The measured values of suspension pH and pHpzc for all the carbons were found to be acidic with more acidic character in the modified carbons. These materials were used as sorbents for the removal of arsenic from aqueous medium and showed excellent adsorption performance.

Abstract: This paper reports the results of the adsorption performance of As(V) removal by a commercial carbon black and its H2SO4-modified form in a single-ion situation. The influence of different process parameters and the physicochemical principles involved were studied in detail. Acid modification caused morphological changes in the virgin carbon black as evidenced by BET surface area measurements and SEM study. FTIR spectra showed the introduction of sulfonic acid group in the parent carbon due to H2SO4 treatment. TGA analysis revealed higher weight loss characteristics of the modified carbon, demonstrating the creation of functional groups. The point of zero charge (pHpzc) of the modified carbon black is highly acidic (3.5) compared to commercial carbon black (6.4). It directly infers the generation of acidic functional moieties in the carbon black. The adsorption experiments were carried out following batch equilibrium techniques. The kinetics and thermodynamics of adsorption were investigated to unveil the mechanism and nature of the adsorption process, respectively. The kinetic parameters of different models were calculated and discussed. The kinetics of adsorption can be expressed by a pseudo-second-order model and intraparticle diffusion was not the rate-determining step. Dependence of pH on adsorption showed maximum metal uptake in the range of 4–5 and inferred surface complexion as the principal mechanism of adsorption. The equilibrium adsorption data were modeled using Freundlich, Langmuir, and Dubinin–Kaganer–Radushkevich (DKR) isotherm equations and the corresponding isotherm parameters were calculated and discussed in detail.

Abstract: The physicochemical factors such as equilibrium time, solution pH, initial concentration of Cd(II), particle size and temperature that control the adsorption of Cd(II) from aqueous solutions onto pyrite has been investigated through batch experiments. Prior to this study, pyrite was characterized through chemical and XRD-analysis. The point of zero charge, pHpzc was determined using the batch equilibrium technique and was found to be 6.4. The equilibrium time was 30 min at the solution pH of 6.0. The pH influence of Cd(II) adsorption was remarkable and maximum metal uptake was observed at 6.0 which is closer to pHpzc. Under this weakly acidic condition Cd(II) ions are responsible for adsorption. Concentration dependence of metal uptake indicates that saturation of pyrite surface by adsorbate occurs at an initial Cd(II) concentration of 350 mg/L and the corresponding metal uptake was 576.5 mg/L of −150 mesh size pyrite at pH 6.0 and 30 °C. Particle size affects the adsorption capacity to a great extent and a decrease in particle diameter enhances metal uptake. The effect of temperature on adsorption performance reveals that the effective temperature for Cd(II) adsorption is 30 °C. The empirical Freundlich isotherm was applied to represent the adsorption process, which fits the experimental data quite well. The work reveals that natural pyrite is a very good choice as an adsorbent for the removal of toxic metals from industrial wastewater and bears significant industrial implications.

Abstract: The desulfurization reaction involving direct electron transfer from potassium ferrocyanide, K4[Fe(CN)6], successfully removed organic sulfur from a subbituminous coal. The temperature variation of desulfurization revealed that increase of temperature enhanced the level of sulfur removal. Moreover, the desulfurization reaction was found to be dependent on the concentration of K4[Fe(CN)6]. Gradual increase in the concentration of K4[Fe(CN)6] raised the magnitude of desulfurization, but at higher concentration the variation was not significant. The removal of organic sulfur from unoxidized coal slightly increased with reduced particle size. Desulfurization from oxidized coals (prepared by aerial oxidation) revealed a higher level of sulfur removal in comparison to unoxidized coal. Highest desulfurization of 36.4 wt % was obtained at 90 C and 0.1 M concentration of K4[Fe(CN)6] in the 100-mesh size oxidized coal prepared at 200 C. Model sulfur compound study revealed that aliphatic types of sulfur compounds are primarily responsible for desulfurization. Because of higher stability, thiophene and condensed thiophene-type of compounds perhaps remained unaffected by the electron-transfer agent. Infrared study revealed the formation of oxidized sulfur compounds (sulfoxide, sulfone, sulfonic acid, etc.) in the oxidized coals. Band intensities in these oxidized compounds due to common -S=O and -SO2 units increased in their respective regions. The desulfurization reaction in different systems is well-represented by the pseudo-first-order kinetic model. The intrinsic rate constants were found to be in the range of (1.8-3.5) × 10-5 s-1, which implies that the reaction proceeds at a slow and steady rate. The activation energy of the reaction in different systems falls in the range of 5.2-8.8 kJ mol-1. Lower values of frequency factor (range: (1.9-5.5) × 10-4 s-1) revealed that during the course of the reaction an associated type of compound (activated complex) was formed. Application of the transition state theory indicated that the desulfurization reaction proceeds with the absorption of heat (endothermic reaction) and is nonspontaneous in nature.