Sorption potential of rice husk for the removal of 2,4-dichlorophenol from aqueous solutions: kinetic and thermodynamic investigations

The sorption potential of chemically and thermally treated rice husk (RHT) for the removal of 2,4-dichlorophenol (DCP) from aqueous solutions has been investigated. Sorption of DCP by rice husk was observed over a wide pH range of 1-10. The effect of contact time between liquid and solid phases, sorbent dose, pH, concentration of sorbate and temperature on the sorption of DCP onto rice husk has been studied. The pore area and average pore diameter of RHT by BET method are calculated to be 17+/-0.6 m2g-1 and 51.3+/-1.5 nm, respectively. Maximum sorption (98+/-1.2%) was achieved for RHT from 6.1x10(-5) moldm(-3) of sorbate solution using 0.1g of rice husk for 10 min agitation time at pH 6 and 303K, which is comparable to activated carbon commercial (ACC) 96.6+/-1.2%, but significantly higher than chemically treated rice husk (RHCT) 65+/-1.6% and rice husk untreated (RHUT) 41+/-2.3%. The sorption data obtained at optimized conditions was subjected to Freundlich, Langmuir and Dubinin-Radushkevich (D-R) isotherms. Sorption intensity 1/n (0.31+/-0.01) and sorption capacity multilayer C(m) (12.0+/-1.6 mmolg(-1)) have been evaluated using Freundlich sorption isotherm, whereas the values of sorption capacity monolayer Q (0.96+/-0.03 mmolg(-1)) and binding energy, b, (4.5+/-1.0)x10(4)dm(3)mol(-1) have been estimated by Langmuir isotherm. The Langmuir constant, b, was also used to calculate the dimensionless factor, R(L), in the concentration range (0.6-6.1)x10(-4) moldm(-3), suggesting greater sorption at low concentration. D-R sorption isotherm was employed to calculate sorption capacity X(m) (2.5+/-0.07 mmolg(-1)) and sorption energy E (14.7+/-0.13 kJmol(-1)). Lagergren and Morris-Weber equations were employed to study kinetics of sorption process using 0.2g of RHT, 25 cm(3) of 0.61x10(-4)moldm(-3) sorbate concentration at pH 6, giving values of first-order rate constant, k, and rate constant of intraparticle transport, R(id), (0.48+/-0.04 min(-1) and 6.8+/-0.8 nmolg(-1)min(-1/2), respectively) at 0.61x10(-4)moldm(-3) solution concentration of DCP, 0.1g RHT, pH 6 and 2-10min of agitation time. For thermodynamic studies, sorption potential was examined over temperature range 283-323 K by employing 6.1x10(-4)moldm(-3) solution concentration of DCP, 0.1g RHT at pH 6 and 10 min of agitation time and values of DeltaH (-25+/-1 kJmol(-1)), DeltaS (-61+/-4 Jmol(-1)K(-1)) and DeltaG(303K) (-7.1+/-0.09 kJmol(-1)) were computed. The negative values of enthalpy, entropy, and free energy suggest that the sorption is exothermic, stable, and spontaneous in nature.     

Batch adsorption and kinetics of chromium (VI) removal from aqueous solutions by Ocimum americanum L. seed pods

In this paper batch removal of hexavalent chromium from aqueous solutions by Ocimum americanum L. seed pods was investigated. The optimum pH and shaker speed were found to be 1.5 and 121 rpm. The equilibrium adsorption data fit well with Langmuir isotherm. The maximum chromium adsorption capacity determined from Langmuir isotherm was 83.33 mg/g dry weight of seed pods at pH 1.5 and shaker speed 121 rpm. The batch experiments were conducted to study the adsorption kinetics of chromium removal for the concentrations of 100 mg/L, 150 mg/L and 200mg/L chromium solutions. The adsorbent dosage was 8 g dry seed pods/L. The removal efficiency observed for all the three chromium concentrations was 100%. The equilibrium was achieved less than 120 min for all the three concentrations. The adsorption kinetic data was fitted with first and second order kinetic models. Finally it was concluded that the chromium adsorption kinetics of O. americanum L. seed pods was well explained by second order kinetic model rather than first order model.     

Application of dithiocarbamate-modified starch for dyes removal from aqueous solutions

The present study shows that the dithiocarbamate-modified starch (DTCS) is a commercially promising sorbent for the removal of anionic dyes from aqueous solutions. It is more effective than activated carbon for this purpose. At the appropriate solution pH of 4, kinetic studies indicate that the sorption of the dyes tends to follow pseudo-first-order equation. The sorption equilibrium is best described by the Langmuir-Freundlich isotherm model at 298 K. The capacities for individual dyes follow the sequence acid orange 7 > acid orange 10 > acid red 18 > acid black 1 > acid green 25, which is consistent with the inverse order of molecular size. The negative enthalpy change for the adsorption process confirms the exothermic nature of adsorption, and a free energy change confirms the spontaneity of the process. The FT-IR spectra and thermogravimetric analyses verify the sorption based on starch-NH(2)(+)CSSH?(-)O(3)S-dye electrostatic attraction. The DTCS can be regenerated from the dye loaded DTCS in a weak basic solution containing sodium sulfate.     

Screening of new sorbent materials for cadmium removal from aqueous solutions

This study compares the abilities of four low-cost materials: peels of peas, broad bean, and medlar, respectively and fig leaves, to remove cadmium from aqueous solutions. Kinetic data and equilibrium sorption isotherms were measured in batch conditions. Kinetics of cadmium sorption was contact time, initial cadmium concentration and sorbent type dependent. The results also showed that the kinetics of cadmium sorption were described by a pseudo second-order rate model. The cadmium uptake of these low-cost materials was quantitatively evaluated using sorption isotherms. Results indicated that Langmuir model gave an acceptable fit to the experimental data. A high cadmium sorption was observed by these materials. The broad bean peel was the most effective to remove cadmium ions with a maximum sorption capacity about 147.71 mg/g followed by peas peel (118.91 mg/g), fig leaves (103.09 mg/g), and medlar peel (98.14 mg/g).     

Sorption of uranium(VI) from aqueous solutions by akaganeite

Removal of U(VI) ions from aqueous solutions was investigated using synthetic akaganeite-type nanocrystals. Nanocrystals of iron oxyhydroxides were synthesized with two different methods and then compared their adsorption capacities. Akaganeite (β-FeOOH) was synthesized in the laboratory by precipitation from aqueous solution of Fe(III) chloride and different precipitating agents. The relative importance of test parameters like solution pH, contact time, temperature and concentration of adsorbate on adsorption performance of akaganeite for U(VI) ion were studied. Typical adsorption isotherms (Langmuir, Freundlich, Dubinin-Raduskevich) were determined for the mechanism of sorption process. Also the thermodynamic constants (ΔH°, ΔS° and ΔG°) were calculated. The product materials were examined by powder X-ray diffraction for crystalline phase identification and scanning electron microscope (SEM).     

Utilization of organic by-products for the removal of organophosphorous pesticide from aqueous media

Sorption potential of rice (Oryza sativa) bran and rice husk for the removal of triazophos (TAP), an organophosphate pesticide, has been studied. The specific surface area were found to be 19+/-0.7 m(2)g(-1) and 11+/-0.8m(2)g(-1) for rice bran and rice husk, respectively. Rice bran exhibited higher removal efficiency (98+/-1.3%) than rice husk (94+/-1.2%) by employing triazophos solution concentration of 3 x 10(-5) M onto 0.2 g of each sorbent for 120 min agitation time at pH 6 and 303 K. The concentration range (3.2-32) x 10(-5) M was screened and sorption capacities of rice bran and rice husk for triazophos were computed by different sorption isotherms. The energy of sorption for rice bran and rice husk was assessed as 14+/-0.1 and 11+/-0.2 kJ mol(-1) and kinetics of the sorption is estimated to be 0.016+/-0.002 and 0.013+/-0.002 min(-1), respectively. Intraparticle diffusion rate was computed to be 4+/-0.8 and 4+/-0.9 nmol g(-1)min(-1/2). Thermodynamic constants DeltaH, DeltaS and DeltaG at varying temperatures (283-323 K) were also calculated.     

Thorough removal of inorganic and organic mercury from aqueous solutions by adsorption on Lemna minor powder

Sorption of the cationic surfactant, hexadecyltrimethylammonium cations (HDTMA), on the solid/liquid interface of the natural mordenite tuff (MT) was studied. The examined tuff originated from Croatia consisting of 30% of mordenite. SEM observations confirmed the crystalline nature of mordenite which can be described in terms of aggregates of many small platelets with diameters in the range of 1 μm. Studying the porosity properties of MT, it was found that the average pore diameter (4.42 nm) between mordenite's platelets allows penetration of HDTMA cations. The measurements of zeta potential indicated that in MT samples with surfactant concentration in the range between 0.013 and 0.25 mmol/g, HDTMA cations fill the mesopores of MT. By further increase in HDTMA concentration, the surfactant sorbs on the external zeolite surface, as revealed by the SEM micrographs. Vibrational (FTIR and FT Raman) spectra showed that in the MT samples with initial HDTMA concentration from 0.013 to 0.25 mmol/g, alkyl chains adopt mainly gauche conformation, whereas in the MT samples with higher initial HDTMA concentrations trans conformers are predominant and form a highly ordered structure on the mordenite surface.     

Application of the hybrid complexation-ultrafiltration process for metal ion removal from aqueous solutions

Complexation-ultrafiltration process was investigated for mercury and cadmium removal from aqueous solutions by using poly(acrylic acid) sodium salt (PAASS) as a complexing agent. The kinetics of complexation reactions of PAASS with the metal ions were studied under a large excess PAASS and pH 5.5. It takes 25 and 50 min for mercury and cadmium to get the complexation equilibrium, respectively, and the reaction kinetics can be described by a pseudo-first-order equation. Effects of various operating parameters such as loading ratios, pH values, etc. on metal rejection coefficients (R) were investigated. In the process of concentration, membrane fluxes decline slowly and R values are about 1. The concentrated retentates were used further for the decomplexation. The decomplexation ratio of mercury-PAASS complex is about 30%, whereas that of cadmium-PAASS complex reaches 93.5%. After the decomplexation, diafiltration experiments were carried out at pH 2.5. Cadmium can be diafiltrated satisfactorily from the retentate, but for mercury it is the contrary. Selective separation of the both metal ions was studied from a binary solution at pH 5. When mercury, cadmium and PAASS concentrations are 30, 30 and 40 mg L(-1), respectively, mercury is retained by ultrafiltration while almost all cadmium passes through the membrane.     

Use of waste materials--Bottom Ash and De-Oiled Soya, as potential adsorbents for the removal of Amaranth from aqueous solutions

Bottom Ash, a power plan t waste material and De-Oiled Soya, an agriculture waste product were successfully utilized in removing trisodium 2-hydroxy-1-(4-sulphonato-1-naphthylazo)naphthalene-3,6-disulphonate--a water-soluble hazardous azo dye (Amaranth). The paper incorporates thermodynamic and kinetic studies for the adsorption of the dye on these two waste materials as adsorbents. Characterization of each adsorbent was carried out by I.R. and D.T.A. curves. Batch adsorption studies were made by measuring effects of pH, adsorbate concentration, sieve size, adsorbent dosage, contact time, temperature etc. Specific rate constants for the processes were calculated by kinetic measurements and a first order adsorption kinetics was observed in each case. Langmuir and Freundlich adsorption isotherms were applied to calculate thermodynamic parameters. The adsorption on Bottom Ash takes place via film diffusion process at lower concentrations and via particle diffusion process at higher concentrations, while in the case of De-Oiled Soya process only particle diffusion takes place in the entire concentration range.     

Application of DNA condensation for removal of mercury ions from aqueous solutions

DNA has a unique character that allows it to combine with various chemical substrates at the molecular level, and the DNA binding with chemical pollutants can cause serious damage to the organism. The purpose of this research was to apply the strong bonding character of DNA for the removal of mercury ions. In this research, we used DNA condensation promoted by the action of DNA condensing agents, such as cetyltrimethylammonium bromide and a commercially available combination flocculant made of zeolite, to precipitate out the DNA bound with mercury ion in an aqueous solution. When solutions of mercury at 0.02–100 ppm (parts per million) concentrations at a pH range of 2–11 were treated with double-stranded DNA followed by the condensing agent, more than 95% of the mercury ions could be removed after simple filtration or sedimentation.     

Arsenate removal from aqueous solutions using modified red mud

Red mud (RM), a waste tailing from alumina production, was modified with FeCl(3) for the removal of arsenate from water. The RM and modified red mud (MRM) were characterized using scanning electron microscopy (SEM) and X-ray diffraction (XRD) microanalysis. Adsorption of arsenate on modified red mud (MRM) was studied as a function of time, pH, and coexisting ions. Equilibrium time for arsenate removal was 24h. Solution pH significantly affected the adsorption, and the adsorption capacity increased with the decrease in pH. Langmuir and Freundlich isotherms equation were used to fit the adsorption isotherms. The Langmuir isotherm was the best-fit adsorption isotherm model for the experimental data. Adsorption capacity of MRM was found to be 68.5mg/g, 50.6 mg/g and 23.2mg/g at pH 6, 7 and 9, respectively. NO(3)(-) had little effect on the adsorption. Ca(2+) enhanced the adsorption, while HCO(3)(-) decreased the adsorption. MRM could be regenerated with NaOH, and the regeneration efficiency reached 92.1% when the concentration of NaOH was 0.2 mol/L.     

Studies on sorption of some geomaterials for fluoride removal from aqueous solutions

In the present study the defluoridation capacities of some of the naturally occurring materials like low and high iron containing lateritic ores, overburden from chromite mines of Orissa Mining Corporation (OMC) and Tata Steel have been estimated. The various experimental parameters studied for fluoride sorption from aqueous solutions were: time, pH, initial fluoride concentration, sorbent dose and temperature. The three geomaterials, namely chromite overburden from Orissa Mining Corporation, both low and high iron containing lateritic ores sorbed fluoride effectively. The sorption kinetics for these samples was found to follow first order rate expression and the experimental equilibrium sorption data fitted reasonably well to both Langmuir and Freundlich models. The negative values of ΔG° suggest the sorption of fluoride onto three samples to be spontaneous and the exothermic nature of sorption is confirmed by the −ΔH° values. The negative ΔS° values for these sorbents point towards decreased randomness at the solid/solution interface. The sorption studies were also carried out at natural pH conditions for fluoride removal from ground water samples and the fluoride level could be reduced from 10.25 to <1.0 mg L⁻¹ by multistage adsorption process using OMC and NH samples.     

Application of natural zeolite for phosphorus and ammonium removal from aqueous solutions

Removal of both nutrients ammonium and phosphorus by natural zeolite has been studied in lab scale by using a mechanically stirred batch system (1000 ml). Zeolite, a mean particle size of 13 μm, was used as an adsorbent for the removal of ammonium and then as a seed material for the precipitation of calcium phosphate. A relationship was established between the uptake of ammonium by zeolite and the ratio of initial ammonium concentration to zeolite dosage. Ammonium uptake of zeolite was almost completed within initial 5 min of adsorption period. There is no pronounced effect of zeolite and ammonium, neither positive nor negative on the amount of calcium phosphate precipitation. The extent of the precipitation of phosphate increased with rising pH. It was also observed that when the system was allowed to relax at constant pH (i.e. under relatively low super saturations), a certain lag time was noted to elapse at the onset of the precipitation. At the pH 7.2, the amount of initial fast precipitation within 5 min and total precipitation within 120 min were around 34% and 93%, respectively. Precipitation of calcium phosphate on to ammonium-loaded zeolite was achieved at low super saturations (<pH 7.5) through secondary nucleation and crystal growth, leading to an increase in particle size.     

Studies on sorption of some geomaterials for fluoride removal from aqueous solutions

In this study, three technologies classified as Advanced Oxidation Processes (Conductive-Diamond Electrochemical Oxidation (CDEO), ozonation and Fenton oxidation) have been compared to treat wastes produced in fermentation processes, and characterized by a significant color and a high organic load. Results of CDEO seem to strongly depend on the addition of an electrolyte salt, not only to decrease the energy cost but also to improve efficiency. The addition of sodium chloride as supporting electrolyte improves the removal percentages of organic load, indicating the important role of mediated oxidation processes carried out by the electrogenerated oxidants (hypochlorite). Fenton oxidation and ozonation seem to be less efficient, and mainly Fenton oxidation favors the accumulation of refractory compounds. The differences observed can be explained in terms of the contribution of hydroxyl radicals and other specific oxidation mechanisms involved in each technology.     

Sorption of U(VI) from aqueous solutions with carbon sorbents

The sorption characteristics of ordinary and oxidized sorts of synthetic (SKN) and kernel (KAU) carbons and also carbon fabric oxidized with HNO₃ (AUTo) with respect to U(VI) were studied. The influence of solution pH on the sorption capacity of carbon materials with respect to uranium was elucidated. The influence of chlorine and sulfate anions on the sorption rate and sorption capacity was studied. Based on kinetic curves and sorption isotherms of uranyl ions and their derivatives, possible mechanisms of uranium adsorption with carbon sorbents were considered. It was shown that carbon sorbents can be used for treatment of aqueous media, among them drinking water, to remove U(VI) compounds.     

Sorption behavior of nano-TiO2 for the removal of selenium ions from aqueous solution

Titanium dioxide nanoparticles were employed for the sorption of selenium ions from aqueous solution. The process was studied in detail by varying the sorption time, pH, and temperature. The sorption was found to be fast, and to reach equilibrium basically within 5.0 min. The sorption has been optimized with respect to the pH, maximum sorption has been achieved from solution of pH 2–6. Sorbed Se(IV) and Se(VI) were desorbed with 2.0 mL 0.1 mol L⁻¹ NaOH. The kinetics and thermodynamics of the sorption of Se(IV) onto Nano-TiO₂ have been studied. The kinetic experimental data properly correlate with the second-order kinetic model (k₂ = 0.69 g mg⁻¹ min⁻¹, 293 K). The overall rate process appears to be influenced by both boundary layer diffusion and intraparticle diffusion. The sorption data could be well interpreted by the Langmuir sorption isotherm. The mean energy of adsorption (14.46 kJ mol⁻¹) was calculated from the Dubinin–Radushkevich (D–R) adsorption isotherm at room temperature. The thermodynamic parameters for the sorption were also determined, and the ΔH⁰ and ΔG⁰ values indicate exothermic behavior.     

A novel agricultural waste adsorbent for the removal of cationic dye from aqueous solutions

In this paper, pineapple stem (PS) waste, an agricultural waste available in large quantity in Malaysia, was utilized as low-cost adsorbent to remove basic dye (methylene blue, MB) from aqueous solution by adsorption. Batch mode experiments were conducted at 30 degrees C to study the effects of initial concentration of methylene blue, contact time and pH on dye adsorption. Equilibrium adsorption isotherms and kinetic were investigated. The experimental data were analyzed by the Langmuir and Freundlich models and the isotherm data fitted well to the Langmuir isotherm with monolayer adsorption capacity of 119.05mg/g. The kinetic data obtained at different concentrations were analyzed using a pseudo-first-order and pseudo-second-order equation and intraparticle diffusion equation. The experimental data fitted very well the pseudo-second-order kinetic model. The PS was found to be very effective adsorbent for MB adsorption.     

Untreated coffee husks as biosorbents for the removal of heavy metals from aqueous solutions

The objective of this work was to propose an alternative use for coffee husks (CH), a coffee processing residue, as untreated sorbents for the removal of heavy metal ions from aqueous solutions. Biosorption studies were conducted in a batch system as a function of contact time, initial metal ion concentration, biosorbent concentration and pH of the solution. A contact time of 72 h assured attainment of equilibrium for Cu(II), Cd(II) and Zn(II). The sorption efficiency after equilibrium was higher for Cu(II) (89-98% adsorption), followed by Cd(II) (65-85%) and Zn(II) (48-79%). Even though equilibrium was not attained in the case of Cr(VI) ions, sorption efficiency ranged from 79 to 86%. Sorption performance improved as metal ions concentrations were lowered. The experimental sorption equilibrium data were fitted by both Langmuir and Freundlich sorption models, with Langmuir providing the best fit (R2>0.95). The biosorption kinetics was determined by fitting first and second-order kinetic models to the experimental data, being better described by the pseudo-second-order model (R2>0.99). The amount of metal ions sorbed increased with the biosorbent concentration in the case of Cu(II) and Cr(VI) and did not present significant variations for the other metal ions. The effect of the initial pH in the biosorption efficiency was verified in the pH range of 4-7, and the results showed that the highest adsorption capacity occurred at distinct pH values for each metal ion. A comparison of the maximum sorption capacity of several untreated biomaterial-based residues showed that coffee husks are suitable candidates for use as biosorbents in the removal of heavy metals from aqueous solutions.     

Thorough removal of inorganic and organic mercury from aqueous solutions by adsorption on Lemna minor powder

The adsorption ability of duckweed (Lemna minor) powders for removing inorganic and organic mercury (methyl and ethyl mercury) has been studied using cold vapour atomic absorption spectrometry. The optimal adsorption conditions were: (a) the pH value of the solution 7.0 for inorganic and ethyl mercury, 9.0 for methyl mercury, and (b) equilibrium adsorption time 10, 20, and 40 min for inorganic mercury, methyl mercury, and ethyl mercury, respectively. After adsorption by L. minor powder for 40 min, when the initial concentrations of inorganic and organic mercury were under 12.0 μg L⁻¹ and 50.0 μg L⁻¹, respectively, the residual concentrations of mercury could meet the criterion of drinking water (1.0 μg L⁻¹) and the permitted discharge limit of wastewater (10.0 μg L⁻¹) set by China and USEPA, respectively. Thorough removal of both inorganic and organic mercury from aqueous solutions was reported for the first time. The significant adsorption sites were C–O–P and phosphate groups by the surface electrostatic interactions with aqueous inorganic and organic mercury cations, and then the selective adsorption was resulted from the strong chelating interaction between amine groups and mercury on the surface of L. minor cells.