Shashi B. Lalvani

A parametric evaluation of the removal of As(V) and As(III) by carbon-based adsorbents

Abstract The As(V) and As(III) removal efficiency of a char-carbon (CC), derived from fly ash in this laboratory, is compared with those of a commercially available Darco activated carbon (DC) and a carbon produced by arcing of graphite rods (AC). The results indicate that CC and AC adsorbents remove almost equal amounts of As(V) at optimum conditions; however, on a percent basis CC removes more As(III) than does AC. In comparison, sample DC was found ineffective for the removal of As(III) and As(V). The present investigation revealed that the adsorption of As(V) onto CC is influenced by pH, initial metal concentration and temperature. Zeta potential measurements were obtained to explain the metal removal behavior of the adsorbents used in this investigation. Since CC shows significant removal efficiency for both As(V) and As(III), there are good prospects for arsenic fixation on CC in practical applications.

Removal of hexavalent chromium and metal cations by a selective and novel carbon adsorbent

Carbon produced by the contact arc method (whereby graphite electrodes are arced in an inert atmosphere) was employed for the removal of hexavalent and trivalent chromium ions as well as other metal cations from aqueous solutions. It is known that hexavalent chromium is present as an anionic species in the solution. The carbon adsorbent used in this study selectively removed the anions of hexavalent chromium from the solution, whereas, depending upon the solution pH, no or very small uptake of metal cations was observed. On the other hand, commercial activated carbon showed great affinity for cations of lead, zinc, and trivalent chromium but none for the anion of hexavalent chromium.

Chromium adsorption by lignin

Hexavalent chromium is a known carcinogen, and its maximum contamination level in drinking water is determined by the U.S. Environmental Protection Agency (EPA). Chromium in the wastewaters from plating and metal finishing, tanning, and photographic industries poses environmental problems. A commercially available lignin was used for the removal of hexavalent as well as trivalent chromium from aqueous solution. It is known that hexavalent chromium is present as an anionic species in the solution. It was found that lignin can remove up to 63% hexavalent and 100% trivalent chromium from aqueous solutions. The removal of chromium ions was also investigated using a commercially available activated carbon. This absorbent facilitated very little hexavalent and almost complete trivalent chromium removal. Adsorption isotherms and kinetics data on the metal removal by lignin and activated carbon are presented and discussed.Hexavalent chromium is a known carcinogen, and its maximum contamination level in drinking water is determined by the U.S. Environmental Protection Agency (EPA). Chromium in the wastewaters from plating and metal finishing, tanning, and photographic industries poses environmental problems. A commercially available lignin was used for the removal of hexavalent as well as trivalent chromium from aqueous solution. It is known that hexavalent chromium is present as an anionic species in the solution. It was found that lignin can remove up to 63% hexavalent and 100% trivalent chromium from aqueous solutions. The removal of chromium ions was also investigated using a commercially available activated carbon. This absorbent facilitated very little hexavalent and almost complete trivalent chromium removal. Adsorption isotherms and kinetics data on the metal removal by lignin and activated carbon are presented and discussed.

Metal Removal from Process Water by Lignin

Lignin obtained as a byproduct from the paper and pulp industry in the form of powder and beads (obtained upon polymerization) was used for the removal of trivalent and hexavalent chromium, lead and zinc from aqueous solutions. Metal uptake as a function of time and temperature was determined. It appears that oxygen functionalities such as phenols present in lignin may serve as sites for the metal cation exchange. The metal bearing lignin upon a wash with 10% sulfuric acid results in elution of copious amounts of metal. The dissolved metals in a concentrated form in the acid can be precipitated by addition of alkali resulting in the metal recovery.

A theoretical approach for predicting AC-induced corrosion

Abstract A model that considers both anodic dissolution and cathodic reduction for corrosion of materials under superimposed sinusoidal voltages has been proposed and developed. A closed form solution permits the estimation of the corrosion current and corrosion potential. The corrosion current is found to increase with the peak voltage of the applied signal. For a constant value of the anodic Tafel slope, a decrease in the cathodic Tafel slope results in higher corrosion current. The corrosion potential is found to be a function of the absolute ratio of the anodic Tafel slope to the cathodic Tafel slope ( r ). For r less than and greater than unity, an increase in the peak voltage results in more active and more noble corrosion potential, respectively, than its DC corrosion potential. Assumptions inherent and the limitations of the model are also discussed in the paper.

A revised model for predicting corrosion of materials induced by alternating voltages

A model that is an improvement upon the previous models for the corrosion of materials under superimposed voltages is proposed. In this model both impedance due to faradaic processes as well as charging current due to the double layer capacitance are considered. Closed form solutions for DC corrosion current and the root-mean-square current are obtained. It is shown that for the model under consideration, the root-mean-square current is a better indicator of the corrosion processes involved than the DC current. The root-mean-square current increases with the peak potential of the voltage and is found to be a minimum at a certain DC potential. The minimum root-mean-square current and the attendant DC potential are found to be a function of the absolute ratio of the anodic Tafel slope to the cathodic slope (r). For example, for r less than and greater than unity, the DC potential at which the root-mean-square current is a minimum is, respectively, more active and more noble than the DC corrosion potential. For r equal to unity, the DC potential at which the root-mean-square current is a minimum is equal to the DC corrosion potential. The total r.m.s. current is found to increase with the frequency of the superimposed alternating voltage and the Tafel slope ratio, r.

The corrosion of carbon steel in a chloride environment due to periodic voltage modulation: Part I

Abstract The influence of negative half-cycle rectified and full-wave sinusoidal potentials superimposed at two DC levels on carbon steel (1018) in 3.33% NaCl solution under nitrogen purge on materials degradation behavior was investigated. The results obtained indicate that the metal dissolution rate increases with the peak potential for the two signals applied at 60 Hz and a fixed potential of −580 mV(SCE). The dissolution rate was found to be the highest for the full-wave sinusoidal and the lowest for the negative half-cycle rectified signals. The metal dissolution rate decreases for the negative half-cycle rectified, but increases albeit moderately for the full-wave sinusoidal potential with the peak potential for experiments conducted at a potential of −780 mV(SCE). Generally, the current was found not to decrease with time except for the experiment involving application of signals at a more cathodic potential of −780 mV(SCE). Micrographs show that the samples experienced extensive pitting. Energy dispersive X-ray analysis data show that the chloride content in corrosion products increases as the metal dissolution rate increases. The data indicate that the arithmetic sum of materials degradation rates for the positive half-cycle and negative half-cycle rectified potentials approximately equals the rate for the full-wave sinusoidal potential. The data obtained strongly indicate that although the corrosion is enhanced by the application of alternating voltages, the rate is generally determined by the DC potential maintained at the anode; the more anodic the potential, the higher the corrosion rate.

Organic sulfur removal from coal by electrolysis in alkaline media

Coal slurries were electrolysed in a basic electrolyte (1 M NaOH) in a batch reactor. Platinum gauze electrodes were employed which were separated by a porous teflon frit that confined coal particles to the anode compartment. A three-electrode potentiostatic technique was used to measure rates of reaction. The amount of sulfur removed was found to be strongly dependent on the electrode potential applied. Over 60% of the total sulfur content can be removed by electrolysis. Moreover, it appears higher cell potentials promote organic sulfur removal (70% at 1.8 to 2.0 V), while lower cell potentials promote pyritic sulfur removal (85% at 1.2 to 1.4 V). In addition, hydrogen gas was observed to be produced at the cathode at about half the potential required for conventional water electrolysis, with current efficiencies higher than 96%. The anodic current, and reaction rates increased monotonically with temperature. An apparent activation energy of 12.05 kcal/mol was calculated for coal oxidation in 1M NaOH solution at an applied potential of 1.2 V versus SCE.

Passivation of pyrite due to surface treatment

Abstract Two samples of naturally occurring pyrite were treated with acetyl acetone, silanes, humic acids, lignin-NaOH mixture, and oxalic acid. The corrosion rate of pyrite was observed to decrease significantly due to the surface treatment by the chemicals used in this study. Surface characterization methods indicate that phenyl triacetoxy silane forms a passive coating a few Angstroms thick on the pyrite, thus preventing its dissolution in 1M H2SO4.

A periodic voltage modulation effect on the corrosion oF Cu-Ni Alloy

Abstract It is suggested in the literature that the application of A.C. fields results in enhanced corrosion rates of materials while the mechanism of corrosion is changed. In this study, corrosion caused by sinusoidal alternating voltage (A.V.) fields in Cu-10Ni alloy (CDA706) in simulated seawater is investigated by: (i) immersion tests and (ii) characterization of corrosion products. The experimental data show that under the influence of alternating voltage (60 Hz) applied between Cu-Ni alloy and a platinum mesh counter electrode, the corrosion rate of the alloy increases with the applied voltage. It was also found that the corrosion rate of the alloy decreased with frequency and time. D.C. polarization curves revealed that passivity was insignificant in the alloy/seawater corrosion system studied. A cuprous oxide film which acts as a diffusion barrier layer to the dissolution of metals protected the alloy from corrosion. Preferred dissolution of nickel was the major anodic reaction causing pitting. The data suggest that application of A.V. signals does not change the mechanism of corrosion of Cu-Ni alloy in simulated seawater.