Sorption of palladium on carbon adsorbents from nitric acid solutions

Sorption recovery of palladium from nitric acid solutions on carbon adsorbents BAU, LKAU-7, ABG and UC has been investigated using model solutions with concentrations 8 × 10⁻⁴–8 × 10⁻³ mol/l for palladium and 1, 2 and 5 mol/l for nitric acid. The recovery degrees of Pd(II) depend on the concentration of palladium in contacting solutions as well as on the type of sorbent used. On average, they reach 60%–100% with the maximum in 1 M HNO₃ The palladium desorption by 10% thiocarbamide solution in 1M H₂SO₄ proceeds completely for the sorbent LKAU-7. The use of thiocarbamide solutions in 0.1 M NaOH increases the desorption of palladium from the sorbents BAU and UC up to 80%–85%     

Performance of acrylic monomer based terpolymer/montmorillonite nanocomposite hydrogels for U(VI) removal from aqueous solutions

Acrylic monomer based terpolymer/montmorillonite nanocomposite hydrogels (NH-MMTs) synthesized using 2-(N,N-dimethylamino)ethyl methacrylate (DMAEMA), 2-acrylamido-2-methlypropane sulfonic acid (AMPS) and 2-hydroxyethyl methacrylate (HEMA) in the aqueous montmorillonite (MMT) suspension were employed as adsorbents for U(VI) removal from aqueous solutions. Adsorption efficiency of the NH-MMTs was strongly enhanced by increasing pH in the range of 3–6. Adsorption capacity of the NHs increased with the MMT weight ratio up to 1% and the complete removal of U(VI) from 1 mmol/L aqueous solutions was achieved by 2 g/L polymer but further increase of MMT up to 6% caused a gradual decrease in adsorption percentage up to 57%. Nearly 98% of U(VI) loaded on the adsorbents could be recovered by 0.1 M HNO₃. Consecutive adsorption/desorption cycles showed that the NH-MMTs are re-usable. Kinetic results were analyzed using Paterson's and Nernst Planck approximation's based on homogeneous solid phase diffusion (HSPD). Experimental data were fitted to equilibrium isotherm models, Langmuir, Freundlich, Dubinin–Radushkevich and Temkin. SEM, and FTIR analysis of bare and U(VI) loaded adsorbents were used to elucidate adsorption mechanisms. The results showed that the NH-MMTs tested in this study are very promising for the recovery of U(VI) from water.     

Sorption of platinum and rhodium on carbon adsorbents from chloride solutions

Sorption recovery of platinum (II, IV) and rhodium (III) on different carbon adsorbents has been studied using fresh chloride model solutions with platinum and rhodium concentrations of 0.25–1.0 and 0.049 mmol/L, respectively. The concentration of hydrochloric acid in these solutions varied from 0.001 to 1.0 mol/L. The maximum recovery of platinum and rhodium was achieved with the carbon adsorbent based on charcoal (wood). This sorbent possesses the highest sorption ability to noble metals in medium acidic solutions (0.01–0.1 M HCl). However, the sorption from higher acid solutions (0.5–1.0 M HCl) proceeds on sufficiently high level as well (>80%). The use of thiocarbamide (10%) solutions in sulfuric acid (0.3 mol/L) as a desorption agent results in almost complete elution of rhodium (more than 95%), whereas platinum is retained in carbon adsorbent. This brings out some prospects for separation of these noble metals during their recovery from solutions of spent platinum–rhodium catalysts.     

The electroreduction kinetics of silver sulfite complexes

The electroreduction kinetics of silver sulfite complexes was investigated by rotation disk electrode (RDE) voltammetry, chronopotentiometry (CP) and electrochemical impedance spectroscopy (EIS). The stability constants of the silver sulfite complexes, pβ₂ = 7.9 and pβ₃ = 8.53 were determined. For the series of isopotential solutions investigated, a reaction order of 0.67 was obtained, the diffusion coefficient of the silver complexes varies in the range of 3.36 × 10⁻⁶ to 5.54 × 10⁻⁶ cm² s⁻¹ and the silver degree of complexation (2.31-2.67) were found. The analysis of the RDE, CP data and EIS spectra indicate the existence of a slow stage of the silver electrocrystallization in the region of the equilibrium potential and at stronger polarization of the electrode at initial time moments.     

Gold dissolution in acidic thiourea and thiocyanate solutions

Gold dissolution in acidic solutions containing thiourea (Tu) and thiocyanate was studied using linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS) and surface enhanced Raman spectroscopy (SERS). A synergistic response found in that mixed ligand system promoted a higher dissolution rate than either lixiviant alone. The passivation of gold occurs in a Tu only solution, but when thiocyanate is mixed with Tu, passivation is significantly alleviated. The dissolution rate of gold in the mixed lixiviant system increases with increasing Tu and thiocyanate concentration. Results from LSV and EIS indicate that gold dissolution is controlled by a combination of charge transfer and diffusion in the mixed lixiviant system. The optimum concentration for Tu and thiocyanate is about 5 mM and 0.05 M, respectively for the rate of gold dissolution. SERS results suggest a possible formation of a mixed ligand complex involving the interaction of Au(Tu)₂⁺ and SCN⁻. Further study is necessary to identify the mixed ligand complex.     

Correlation between retention and adsorption of phenolic compounds in nanofiltration membranes

Equilibrium adsorption experiments of phenol, 3-chlorophenol, 4-chlorophenol, and 3-nitrophenol aqueous solutions on NF90 membrane were conducted to obtain the corresponding adsorption isotherms at 25 ºC. Single-compound solutions with concentration ranging from 0.1 to 8 mmol L^− 1 were used. Freundlich and Langmuir models were compared to the experimental isotherms and their characteristic parameters were obtained from linear fits. In addition, the adsorptive behaviour of twelve aqueous phenolic compounds on the NF90 membrane was studied in order to investigate the relationship between adsorption and retention of selected solutes. An inverse correlation between the adsorbed amount, at the same equilibrium concentration (1 mmol L^− 1), and retention was found. The influence of the molecular hydrophobicity and dipole moment of phenolic compounds on membrane adsorption, solute retention and water flux decline was also investigated.     

Experimental design and batch experiments for optimization of Cr(VI) removal from aqueous solutions by hydrous cerium oxide nanoparticles

Hydrous cerium oxide (HCO) was synthesized by intercalation of solutions of cerium(III) nitrate and sodium hydroxide and evaluated as an adsorbent for the removal of hexavalent chromium from aqueous solutions. Simple batch experiments and a 2⁵ factorial experimental design were employed to screen the variables affecting Cr(VI) removal efficiency. The effects of the process variables; solution pH, initial Cr(VI) concentration, temperature, adsorbent dose and ionic strength were examined. Using the experimental results, a linear mathematical model representing the influence of the different variables and their interactions was obtained. Analysis of variance (ANOVA) demonstrated that Cr(VI) adsorption significantly increases with decreased solution pH, initial concentration and amount of adsorbent used (dose), but slightly decreased with an increase in temperature and ionic strength. The optimization study indicates 99% as the maximum removal at pH 2, 20 °C, 1.923 mM of metal concentration and a sorbent dose of 4 g/dm³. At these optimal conditions, Langmuir, Freundlich and Redlich–Peterson isotherm models were obtained. The maximum adsorption capacity of Cr(VI) adsorbed by HCO was 0.828 mmol/g, calculated by the Langmuir isotherm model. Desorption of chromium indicated that the HCO adsorbent can be regenerated using NaOH solution 0.1 M (up to 85%). The adsorption interactions between the surface sites of HCO and the Cr(VI) ions were found to be a combined effect of both anion exchange and surface complexation with the formation of an inner-sphere complex.     

Adsorption and desorption of DMF on macroporous resin NKA-II in the fixed bed

To reduce the high energy consumption during the traditional ordinary distillation process for recycling N,N-dimethyl formamide (DMF), this paper utilized the NKA-II macroporous adsorptive resin in combination with a distillation process to recycle DMF in wastewater. First, the adsorption equilibrium data were measured in the intermittent agitation tank, which showed that the DMF adsorption equilibrium on the NKA-II resin complies with the Henry equation. The dynamic experimental studies indicated that the adsorption temperature has little effect on the adsorption process; the flow rate and the bed height affect the breakthrough time but have little effect on the mass transfer zone. With the combination of the fixed-bed adsorption model and the breakthrough curve, the surface diffusion coefficient of the DMF on the resin in the fixed bed was approximately 3.50 × 10⁻¹⁰ to 1.06 × 10⁻⁹ m² s⁻¹. The simulated values were in good agreement with the breakthrough curves determined by experiments. Furthermore, ethanol was selected as a better desorption agent. The appropriate desorption conditions were determined to be a flow rate of 8.00 × 10⁻⁵ m s⁻¹ to 1.58 × 10⁻⁴ m s⁻¹ and a temperature of 308–318 K. Under these conditions, the desorption rates were all greater than 99%. Finally, wastewater that contained 5% DMF was used as an example to analyze the energy consumption. The results indicated that the adsorption–distillation process can reduce the energy consumption by 79%. The adsorption–distillation process has a good applicable value for the recovery of DMF in wastewater, especially for wastewater with a low concentration of DMF.     

Effect of sulphuric acid and copper sulphate concentrations on the morphology of silver deposit in the cementation process

The silver ion cementation on copper was investigated in the presence or absence of oxygen in solutions containing 1.85 × 10⁻⁴ M Ag⁺ at 25 °C. The influence of sulphuric acid and copper sulphate concentration (0.005-0.5 M) on the silver cement morphology was studied in details and results were linked with the previously determined kinetics data of the process. The morphology of silver deposit was found to be independent of the presence of oxygen in the system as well as the sulphuric acid concentration. Contrary, the concentration of copper sulphate strongly influenced the morphology of silver deposit. At the beginning of the cementation process silver covers uniformly the copper surface. Afterwards, a growth of dendrites is initiated on preferential parts of the surface. The growing dendrite behaves as cathodic sites, with relatively huge surface area and promotes the creation of anodic sites in a close neighbourhood. Finally, the anodic site encloses the dendrite island and develops its area inward the copper material. Copper ions at low concentration modified slightly silver dendrites but the increase in concentration up to 0.5 M Cu²⁺ leads to completely disappearance of dendrites from the surface. The lack of dendrites on the surface is a result of the competitive process that consumes additional silver ions, occurring in the bulk of the solution. The morphology of silver deposit cemented in the deoxygenated solution containing 0.5 M H₂SO₄ + 0.5 M CuSO₄ depends strongly on the mechanism of the process.     

Development of solid-phase microextraction for the determination of trihalomethanes in drinking water from Bizerte, Tunisia

Headspace-solid-phase microextraction (HS-SPME) combined with gas chromatography-electron capture detector (GC-ECD) has been developed and studied for the determination of trihalomethanes (THMs) in treated water samples. Experimental parameters such as the selection of thickness of the polymer coating, addition of salt, magnetic stirring, extraction temperature, and extraction time were studied. Extraction of the analytes was performed using HS-SPME with a 100 µm poly(dimethylsiloxane) coating followed by thermal desorption at 250 °C and GC analysis. The optimized conditions were 20 min extraction time at 35 °C with 25 w/v% NaCl. Analytical parameters such as linearity and limit of detection were also evaluated. The linear range of 1-100 µg/l was established with relative standard deviations (%RSD) within the range, 1.3-11.7%. The limits of detection (LODs) were ranged from 1.4 ng/l to 6.1 ng/l. The average THM concentration was 88.16 µg/l which was well within the proposed European Union directive of 100 µg/l.     

Electrochemical recovery of silver from cyanide leaching solutions

In the hydrometallurgy industry cyanide solutions are the most common leaching baths used during the extraction of metals such as silver and gold. After extraction, the solution containing various cyanide species, and usually copper cyanide, has a higher concentration than the gold and silver complexes. Higher concentration of copper species may interfere during the selective recovery of precious metals. This work presents a study of the selective recovery of silver from leaching solutions mimicking those used in industry. Chemical speciation and cyclic voltammetry studies showed that copper reduction occurs at more negative potentials than silver in the cyanide leaching solutions. The cyclic voltammetry of cyanide solutions on a vitreous carbon electrode showed that copper cyanide ions modify the interface properties, lowering the overpotential required for silver reduction. A macroelectrolysis study of a simulated leaching solution (10^–4 M Ag(I), 0.1 M Cu(I) in 0.5 M CN^– at pH 10), was carried out in a filter press electrochemical reactor (ElectroCell AB) with a reticulated vitreous carbon electrode (RVC), nominally having 45 pores per inch and a flow rate of 5 cm³ s^–1 at 25 °C. The study showed that the high copper concentration does not interfere in the selective deposition of silver.     

Detection of synthetic oligonucleotides by alternating current voltammetry at solid amalgam surfaces

In this paper we investigate the tensammetric responses of different synthetic oligonucleotides (ODNs) by an alternating current (AC) voltammetry at the platinum solid amalgam electrode (PtAE). The optimisation of the AC voltammetric measurement of ODNs includes the determination of the effects of: adsorption time (t_A), concentration of analysed sample (c), thickness of the amalgam layer (h), and use of different solid amalgams such as platinum, copper, gold and silver on the intensity of the AC voltammetric peaks 1 (desorption of ODN segments adsorbed via ODN sugar-phosphate backbone) and 3 (desorption and/or reorientation of the ODN segments adsorbed through hydrophobic base residues) of adsorbed ODNs. The features of mercury modified graphite/carbon electrodes for measurements of tensammetric responses of ODNs are briefly discussed. We show that from the height of the AC voltammetric peak 3 and potential difference between the AC voltammetric peaks 1 and 3 it is possible to identify: (a) different homopurine ODN lengths, and (b) a proportion of purine (adenine and/or adenine + guanine) units to the whole length of different ODNs containing both the purine and pyrimidine units.     

Adsorption of thiourea on monocrystalline silver electrodes in neutral solution

Impedance spectroscopy and radiometric method have been used in the study of thiourea (TU) adsorption on monocrystalline silver electrodes of basal indices: (1 1 1), (1 0 0) and (1 1 0) in neutral solution. The dependence of the surface concentration of TU on the electrode potential and on the bulk concentration was determined for each studied surface. From radiometric measurements it follows that adsorption of TU on silver electrodes takes place in the entire range of applied potentials. The process of adsorption is practically reversible with respect to the electrode potential (in the range of the double layer) and the bulk concentration of TU. Differential capacity of silver electrodes in 0.01 M NaClO₄ solution containing TU of concentrations from 10⁻⁶ to 5 × 10⁻⁴ M has been measured. The isotherms of TU adsorption, determined from the capacitance and radiometric measurements have been compared and the Gibbs energy of adsorption was calculated. The values of limiting surface concentration of adsorbed TU as well as the Gibbs energy of adsorption depend on the plane of Ag electrode and follow the sequence: Ag(1 1 1) > Ag(1 0 0) > Ag(1 1 0) which is in agreement with the surface density of Ag atoms.     

Determination of picomolar silver concentrations by differential pulse anodic stripping voltammetry at a carbon paste electrode modified with phenylthiourea-functionalized high ordered nanoporous silica gel

This study introduces the design of an anodic stripping voltammetric (ASV) method for the silver ion determination at a carbon paste electrode (CPE), chemically modified with phenylthiourea-nanoporous silica gel (Tu-SBA-15-CPE). The electroanalytical pro includes two steps: preconcentration of metal ions at an electrode surface, followed by quantification of the accumulated species by differential pulse anodic stripping voltammetric methods. Factors affecting the performance of the anodic stripping were investigated, including the modifier quantity in the paste, the electrolyte concentrations, the solution pH and the accumulation potential or time. The most sensitive and reliable electrode contained 10% Tu-SBA-15 and 90% carbon paste. The accumulation potential and time were set at, −200 mV and 300 s, respectively, and the scan rate at 50 mV s⁻¹ in the scan range of −200 to 700 mV. The resulting electrode demonstrated a linear response over range of silver ion concentration of 8.0-80 pmol/L with detection limit (S/N = 3) of 5 pmol/L. The prepared electrodes were used for the silver determination in sea and tap water samples and very good recovery results were obtained. The accuracy was assessed through recovery experiments and independent analysis by graphite furnace atomic absorption spectrometry.     

Adsorption of Hg(II) ions from aqueous chloride solutions using powdered activated carbons

Activated carbons were developed from Casurina equisetifolia leaves, by chemically treating with sulfuric acid (1:1) or zinc chloride (25%), at low (425 °C) and high (825 °C) temperatures. The resulting powdered activated carbons were applied for removing mercuric ions from aqueous solution at different agitation times and mercuric ion concentrations. The equilibrium data fitted well the Langmuir adsorption isotherm. The Langmuir adsorption capacities were 12.3 and 20.3 mg g⁻¹ for low temperature carbons and 43.9 and 38.5 mg g⁻¹ for high temperature carbons impregnated with H₂SO₄ and ZnCl₂, respectively. Studies of the effects of carbon dosage, NaCl concentrations and solution pH values were carried out for the more effective, high temperature carbons. Increasing NaCl concentration resulted in a significant decrease in the adsorption efficiency. Adsorption was high from solutions with low and neutral pH values and lower for solutions with alkaline pH values for the high temperature carbons.     

Advanced oxidation of catechol: A comparison among photocatalysis, Fenton and photo-Fenton processes

The aim of this work was to compare the behaviour of Fenton, photo-Fenton and photocatalysis processes to treat catechol solutions which are pollutants occurring in wastewaters from many industries. The effect of different process parameters, such as initial catechol concentration, H₂O₂/FeSO₄ ratio in Fenton and photo-Fenton oxidation, TiO₂ loadings in photocatalysis and irradiation times has been studied.Fenton and photo-Fenton (H₂O₂/FeSO₄ = 600/500 (w/w) and 30 min reaction time) processes allowed us to achieve a high efficiency in the mineralization of catechol (COD removals up to 83% and 96% respectively), and removal of aromaticity (UV₂₈₀) (up to 93% and 98% respectively), for an initial catechol concentration of 110 mg/l. On the opposite, photocatalysis was not effective in the removal of higher catechol concentrations (110 and 200 mg/l), whereas a significant removal of aromaticity versus time was observed for 50 mg/l. Gas chromatography-mass spectrometry analysis, performed under selected treatment conditions, showed that total removal of catechol can occur after Fenton (2000/500 w/w; 30 min), photo-Fenton (600/500 w/w; 30 min), and photocatalysis (3 g TiO₂/l; 240 min) treatments.     

Size-controlled Bi-based glass powders prepared by spray pyrolysis as inorganic additives for silver electrode

Bi-based glass powders as additive for silver conducting pastes were prepared by spray pyrolysis. The glass powders formed from the spray solution with low concentration of 0.025 M had bimodal size distribution with nanometer and submicron sizes. However, glass powders with spherical shape and narrow size distribution were prepared from the spray solutions with concentrations of 0.05 and 0.5 M. The mean size of the glass powders increased from 0.34 to 0.7 μm when the concentrations of the spray solutions changed from 0.05 to 0.5 M. The glass transition temperature of the glass powders with the mean size of 0.34 and 0.70 μm were 382 and 396 °C, respectively. The glass layers fired at 450 °C had clean surfaces irrespective of the mean size of the glass powders. Silver conducting films were formed by melting of the silver powders irrespective of the mean sizes of the glass powders at firing temperatures between 400 and 500 °C. The specific resistances of the silver conducting films change from 3.13 to 4.03 μΩ cm according to the mean size of the glass powders at a firing temperature of 500 °C.     

Possible interference in the sequential voltammetric determination at trace and ultratrace concentration level of platinum group metals (PGMs) and lead : Application to environmental matrices

The present work describes a procedure for the sequential determination of Pd(II), Pt(II), Rh(III), by square wave adsorption stripping voltammetry (SWAdSV) and Pb(II) by square wave anodic stripping voltammetry (SWASV) in environmental matrices (sediments, soils and superficial water) in the presence of possible metal interferences, including high concentration ratios.The supporting electrolytes were 0.1 mol/L HCl, 0.1 mol/L HCl + 1.8 × 10⁻⁴ mol/L dimethylglyoxime (DMG) and 0.6 mmol/L formaldehyde + 1.2 mmol/L hydrazine (formazone complex) in 0.1 mol/L HCl.The voltammetric measurements were carried out using, as working electrode, a stationary hanging mercury drop electrode (HMDE), a platinum wire as auxiliary and an Ag|AgCl|KCl_sat as reference electrode.The analytical procedure was verified by the analysis of standard reference materials (CCRMP-CANMET-TDB-1 and CCRMP-CANMET-UMT-1 (rock soils), Sea Water BCR-CRM 403 and Fresh Water NIST-SRM 1643d). In the case of water standard reference materials, the solutions were spiked with known element concentrations, successively verifying the percentage recovery.In the presence of reciprocal interference, the standard addition method considerably improved the resolution of the voltammetric technique, even in the case of very high element concentration ratios.Once set up on the standard reference materials, the analytical procedure was transferred and applied to sediments, soils and superficial waters sampled in proximity of superhighway and in the Po River mouth area.A critical comparison with spectroscopic measurements is also discussed.     

Comparison of the adsorption performance of compound adsorbent in a refrigeration cycle with and without mass recovery

Adsorption and desorption were performed on a compound adsorbent composed of CaCl₂ and activated carbon in cycles both with and without mass recovery and the performances obtained were compared with those of equilibrium adsorption. Experimental results of the cycles without mass recovery carried out on an adsorption refrigeration unit yielded performances that were slightly less than those of equilibrium adsorption. The adsorption performances of the cycle with mass recovery were measured to be much better than those of the cycle without mass recovery. At of evaporating temperature, the cycle adsorption capacity was as high as 0.78 kg/kg for the cycle with mass recovery while it was only 0.55 kg/kg for the cycle without mass recovery. The average adsorption/desorption rate of the cycle with mass recovery, which was 0.031 (kg/kg)/min, has been improved by 47.6% compared with that of the cycle without mass recovery. Research on the cycle adsorption capacity at different evaporating temperatures showed that the improvement of cycle adsorption capacity, with mass recovery, was higher under the condition of lower evaporating temperature. At evaporating temperature, the mass recovery operation had improved the adsorption capacity by 78% compared with the cycle without mass recovery. In addition, refrigeration performances for the cycles with and without mass recovery at an evaporating temperature of were studied. Compared with the results of the cycle without mass recovery, SCP (specific cooling power) and COP (coefficient of refrigeration performance) have been improved by 48.6% and 54.5%, respectively, when mass recovery is performed.     

New pathway for the synthesis of gold and silver bimetallic nanocomplexes and their derivatives of polypyrrole nanorods (TM04096)

We report here the first electrochemical pathway to prepare Au- and Ag-containing bimetallic nanocomplexes with a mean diameter of 5 nm in 0.1 M HCl aqueous solutions without addition of any stabilizer. First, a silver substrate was roughened by a triangular-wave oxidation-reduction cycle (ORC) in an aqueous solution containing 0.1 M HCl. Silver-containing complexes were left in the solution after the ORC treatment. Then a gold substrate was subsequently roughened by the similar ORC treatment in this solution. Encouragingly, polypyrrole (PPy)-coated Au and Ag bimetallic nanocomposites with a nanorod structure and a diameter smaller than 15 nm can be prepared by the formation of self-assembled monolayers and orderly autopolymerization of pyrrole monomers on these bimetallic nanocomplexes, and further link them together.