Removal of selected metals from drinking water using modified powdered block carbon

This paper presents the possible alternative removal options for the development of safe drinking water supply in the trace elements affected areas. Arsenic and chromium are two of the most toxic pollutants, introduced into natural waters from a variety of sources and causes various adverse effects on living bodies. Performance of three filter bed method was evaluated in the laboratory. Experiments have been conducted to investigate the sorption of arsenic and chromium on carbon steel and removal of trace elements from drinking water with a household filtration process. The affinity of the arsenic and chromium species for Fe/Fe₃C (iron/iron carbide) sites is the key factor controlling the removal of the elements. The method is based on the use of powdered block carbon (PBC), powder carbon steel and ball ceramic in the ion-sorption columns as a cleaning process. The PBC modified is a satisfactory and practical sorbent for trace elements (arsenite and chromate) dissolved in water.     

Oxidation mechanisms and chemical bioavailability of chromium in agricultural soil – pH as the master variable

Chromium (Cr) is a heavy metal that exists in soils in two stable oxidation states, +III and +VI. The trivalent species is an essential nutrient, whereas the hexavalent species is highly toxic. This study investigated the environmental impact of Cr^III potentially released into soil from wastes and various materials by determining the risk of oxidation of initially soluble inorganic Cr^III into hazardous Cr^VI. The principal aim was to describe the pH-dependent mechanisms that regulate 1) the formation of Cr^VI from the easily soluble Cr^III and 2) the potential bioavailability of Cr^III and that of Cr^VI species produced in the oxidation of Cr^III in agricultural soil (fine sand, organic carbon 3.2%). The amount of Cr^VI formed in oxic soil conditions was regulated by two counteracting reactions: 1) oxidation of Cr^III into Cr^VI by manganese oxide (Mn^IVO₂) and 2) the subsequent reduction of Cr^VI by organic matter back to Cr^III. The effect of pH on this net-oxidation of Cr^III and on the chemical availability of both Cr^III and Cr^VI species was investigated in soil samples incubated with or without excessive amounts of synthetic MnO₂, over the chemically adjusted pH range of 3.9–6.3 (+22 °C, 47 d). In soil subsamples without added MnO₂, the net-oxidation of Cr^III into Cr^VI (1 mM CrCl₃ in soil suspensions, 1:10 w/V) was negligible. As for the MnO₂-treated soils, at maximum only 4.7% of added Cr^III was oxidized – regardless of the high oxidation potential of these subsamples. The lowest production of Cr^VI was observed under acidic soil conditions at pH ∼4. At low pH, the net-oxidation diminished as result of enhanced reduction of Cr^VI back to Cr^III. At higher pHs, the oxidation was limited by enhanced precipitation (or adsorption) of Cr^III, which lowered the overall amount of Cr^III susceptible for oxidation. Moreover, the oxidation reactions by MnO₂ were inhibited by formation of Cr(OH)₃ coverage on its surface. The pH-dependent chemical bioavailability of added Cr^III differed from that of the Cr^VI formed. At elevated pHs the chemical availability of Cr^III decreased, whereas that of Cr^VI produced increased. However, the risk of Cr^VI formation through oxidation of the easily soluble inorganic Cr^III was considered to be low in agricultural soils high in organic matter and low in innate MnO₂.     

Prediction of Gibbs free energies of formation of minerals of the alunite supergroup

A method for the prediction of Gibbs free energies of formation for minerals belonging to the alunite family is proposed, based on an empirical parameter Δ_GO⁼ M^z+(c) characterizing the oxygen affinity of the cation M^z+. The Gibbs free energy of formation from constituent oxides is considered as the sum of the products of the molar fraction of an oxygen atom bound to any two cations, multiplied by the difference of oxygen affinity Δ_GO⁼ M^z+(c) between any two consecutive cations. The Δ_GO⁼ M^z+(c) value, using a weighing scheme involving the electronegativity of a cation in a specific site (12-fold coordination site, octahedral and tetrahedral) is assumed to be constant. It can be calculated by minimizing the difference between experimental Gibbs free energies (determined from solubility measurements) and calculated Gibbs free energies of formation from constituent oxides. Results indicate that this prediction method gives values within 0.5% of the experimentally measured values. The relationships between Δ_GO⁼ M^z+(alunite) corresponding to the electronegativity of a cation in either dodecahedral sites, octahedral sites or tetrahedral sites and known as Δ_GO⁼ M^z+(aq) were determined, thereby allowing the prediction of the electronegativity of rare earth metal ions and trivalent ions in dodecahedral sites and highly charged ions in tetrahedral sites. This allows the prediction of Gibbs free energies of formation of any minerals of the alunite supergroup (bearing various ions located in the dodecahedral and tetrahedral sites). Examples are given for hydronium jarosite and hindsalite, and the results appear excellent when compared to experimental values.     

Remediation of ground water containing chlorinated and brominated hydrocarbons, benzene and chromate by sequential treatment using ZVI and GAC

A laboratory experiment with two sequenced columns was performed as a preliminary study for the installation of a permeable reactive barrier (PRB) at a site where a mixed ground water contamination exists. The first column contained granular zero valent iron (ZVI), the second column was filled with granular activated carbon (GAC). Trichloromethane (TCM, 930 μg/l) and chlorobenzene (MCB, 260 μg/l) were added to the ground water from the site as the main contaminants. Smaller amounts (<60 μg/l) of benzene, 1,2-dichloroethane, 1,1,2-trichloroethane (1,1,2-TCA), 1,1-dichloroethene (1,1-DCE), trichloroethene (TCE), tetrachloroethene (PCE), 1,2-dichloropropane (1,2-DCP), bromodichloromethane (BDCM), dibromochloromethane (DBCM), tribromomethane (TBM), vinyl chloride and chromate were also added to the water to simulate the complex contamination pattern at the site of interest. PCE, TCE, 1,1-DCE, DBCM, BDCM, TBM, MCB and chromate were remediated in contact with ZVI, while the remaining contaminants showed incomplete degradation. A fraction of 8–16.5% TCM was converted to dichloromethane (DCM). Remaining contaminant concentrations were efficiently sorbed by the GAC until breakthrough of DCM was observed after 1,230 exchanged pore volumes in the GAC. The results show that the complex mixture of contaminants can be remediated by a sequenced PRB consisting of ZVI and GAC and that DCM sorption capacity is the critical parameter for the dimensions of the GAC reactor.     

Removal of chromate from drinking water using powder carbon steel

This paper presents the possible alternative removal options for the development of safe drinking water supplies in areas affected by hexavalent chromium. Chromate is a matter of great environmental concern due to its extensive contamination and carcinogenic toxicity. In this study, the conventional adsorption with various types of adsorbent method were used for chromate removal, but only powder carbon steel was compatible with household water treatment and can be designated based on high removal efficiency and affordable cost. Home powder carbon steel with granular activated carbon (PCS–GAC) treatment systems are quite simple. Water needing treatment passes through the cartridge or filter candles contacting PCS–GAC on its way to the faucet. This method is based on the use of powder carbon steel, ML 90 Bombril S.A, as a cleaning agent. Granular activated carbon filters have been employed in home water purification systems essentially for the removal of taste, odor, and color. The effect of pH, redox potential, time, and adsorbate concentration on the uptake carbon steel were observed. The lack of desorption suggests that anion chromium is irreversibly sorbed by the powder carbon steel.     

Chromium speciation and existing natural attenuation conditions in lagoonal and pond sediments in the former chemical plant of Porto-Romano (Albania)

Near the outskirts of the Albanian port of Durres on the Adriatic Coast, at a place called Porto Romano, an old chemical complex has been producing chromate salts for a period of ∼20 years (1972–1993). As a result large quantities of chromite ore processing residue (COPR) waste were released mainly as suspended particles and/or dissolved species in effluents, which settled in a decantation pond area, 200 × 250 m in size, east of the industrial plant. Unfortunately, pollutants were spread out with time. Knowing that chromium mobility and toxicity in natural settings is species dependent, the species characterization has been carried out in different sampling media (pond sediments, lagoonal sediments and water samples). Five pond profiles were sampled within the decantation area up to a depth of ∼1 m, till the underlying Quaternary lagoonal silty-clays were reached. Total Cr content is highly variable with values between 1,130 and 24,409 mg/kg which is clearly higher than the defined local background values for lagoonal silty-clays (198 mg/kg of Cr). Leaching of Cr(VI) especially occurs in the pond sediments, which are low in OC, CEC and clay content, under acidic and neutral pH conditions (e.g. up to ∼2,230 mg/kg Cr(VI) leached from a sample containing a total concentration of Cr 12,200 mg/kg). Moreover, leaching of the Cr(III) occurs only under strictly acidic conditions (maximum 1,144 mg/kg leached from a sample containing a total Cr-concentration of 17,608 mg/kg). In this study also a number of natural attenuation conditions (i.e. reaction with lagoonal clays rich in organic matter and iron as well as isomorphous substitution) have been recognized.     

Removal of chromate from aqueous solution using treated natural zeolite

This paper present the possible alternative removal options for the development of safe drinking water supply in the chromium-affected areas. The Cr (VI) state is of particular concern because of its toxicity. The mordenite has suitable mineralogical properties that enable them to be used for ion-exchange processes. This includes total cation exchange capacity. However, in the present work, the modified-natural zeolite was used as an adsorbent for the removal of Cr (VI) from aqueous solution. The ability of modified natural zeolite (mordenite) to remove inorganic anion was investigated. Laboratory experiments were conducted examining the effect of the sorption of cationic surfactants. On the basis of the results of this study, the HDTMA-HSO₄ modified zeolite appears suitable as a sorbent for hexavalent chromium whereas EHDDMA-modified zeolite were not removed with the same efficiency. The sorption of chromate on HDTMA-zeolite results from a combination of entropic, coulombic, hydrophobic effects, and HDTMA counterion.     

Chromium(VI) reduction by sulphur(IV) in aqueous solutions

The rates of the reduction of Cr(VI) with S(IV) were measured in deaerated NaCl solution as a function of pH, temperature and ionic strength. The rates of the reaction were found to be first order with respect to Cr(VI) and second order with respect to S(IV), in agreement with previous results obtained at concentrations two order higher than the present study. The reaction also showed a first-order dependence of the rates on the concentration of the proton and a small influence of temperature with an apparent energy of activation ΔH_app of 22.8 ± 3.4 kJ/mol. The rates were independent of ionic strength from 0.01 to 1 M. The rate of Cr(VI) reduction is described by the general expression

−d[Cr(VI)]/dt=k[Cr(VI)][S(IV)]²