C.P. Huang

The adsorption of heavy metals onto hydrous activated carbon

The absorption characteristics of some heavy metals, namely CU(II), Pb(II), Ni(II) and Zn(II) onto the hydrous activated carbon surface are studied. Carbon type, pH and surface loading are the most important factors affecting the extent of metal removal. The adsorption reaction can be best described by a surface complex formation model. The free metal ions, M2+, and their hydroxo species, M(OH)y2−y all participate in the absorption reaction. Specific chemical bonding, probably hydrogen bonding provided the sole energy needed for adsorption reaction.

The removal of chromium(VI) from dilute aqueous solution by activated carbon

The removal of chromium(VI) by activated carbon, filtrasorb 400, is brought by two major interfacial reactions: adsorption and reduction. Chemical factors such as pH and total Cr(VI) that affect the magnitude of Cr(VI) adsorption were investigated. The adsorption of Cr(VI) exhibits a peak value at pH 5–6. The particle size of carbon and the presence of cyanide species do not change the magnitude of chromium removal. The reduced Cr(VI), e.g. Cr(III) is less adsorbable than Cr(VI). The free energy of specific chemical interaction, ΔGchem was computed by the Gouy-Chapman-Stern-Grahame model. The average values of ΔGchem are −5.57 RT and −5.81RT, respectively, for Cr(VI) and CN. These values are significant enough to influence the overall magnitude of Cr(VI) and CN adsorption. Results also indicate that HCrO−4 and Cr2O2−7 are the major Cr(VI) species involved in surface association.

Adsorption characteristics of some Cu(II) complexes on aluminosilicates

The adsorption of Cu(II) by aluminosilicates with varying Si/Al ratios was investigated. The presence of complex-forming organic ligands [nitrilotriacetate (NTA) and glycine (Gly)] alters metal electrovalency and, in so doing, modifies Cu(II) adsorption characteristics which can influence its fate, biological activity and transport in aquatic systems. Electrostatic attraction by a positively-charged aluminosilicate surface is an important mechanism whereby anion CuNTA− complexes were adsorbed. Two distinct mechanisms are involved in the adsorption of cationic complexes: (1) an exchange reaction at permanent structural sites and (2) interfacial accumulation in response to the pH-dependent surface charge. The contribution of each mechanism to the total amount of CuGly+ adsorbed is related to the Si/A1 ratio. At the critical Si/A1 ratio (Si/A1iso), the aluminosilicates have zero net pH-dependent surface charge. In the absence of specific adsorption, aluminosilicates for which Si/A1 ≥ Si/A1iso can only function as cation exchangers. For Si/A1 < Si/A1iso simultaneous adsorption of anions and cations is possible.

The removal of mercury(II) from dilute aqueous solution by activated carbon

Abstract The results of preliminary screening tests comparing the total Hg(II) removal capacity of 11 different brands of commercial activated carbon indicated that a very high percent (99–100%) total Hg removal was attained by all types of activated carbon especially at pH 4–5; the percent total Hg(II) removal decreased with pHs ≷4–5 except activated carbons Nuchar SA and SN which maintained a relatively high percent (>90%) total Hg(II) removal capacity at all pH values. Experiments were then conducted to reveal the mechanisms of Hg(II) removal by Nuchar SA (a powdered carbon). The results show that total Hg(II) removal was brought by two mechanisms: the adsorption and reduction. In order to investigate the kinetics of these two reactions, volatilization by bubbling N2 gas at high flow rate was used to remove the Hg(g) product of the reduction reaction. It was noted that both the adsorption and the reduction/volatilization reactions were highly pH-dependent; at pH approx. approx. 9–10 the extent of reduction/volatilization reaction superceded the adsorption reaction; whereas in the mid-pH region adsorption reaction dominated the total Hg(II) removal. The rate of adsorption reaction is very fast, reaching equilibrium in a few minutes; the rate of reduction/volatilization follows a linear √t expression. The reduction reaction is more significant with Filtrasorb 400 (H-type carbon) than Nuchar SA (L-type carbon). In the presence of strong chelating agent, ethylenediaminetetraacetate (EDTA), the total Hg(II) removal decreases due partly to the formation of less adsorbably mercuric(II)-EDTA complexes.

Application of Aspergillus oryze and Rhizopus oryzae for Cu(II) removal

Biosorption of copper(II) by the untreated and acid-treated fungal biomass has been studied in both batch and column modes. Two species, Aspergillus oryzae and Rhizopus oryzae, were used in this study. A. oryzae mycelia (undergoing acid-washing) exhibit a clear advantage in Cu(II) removal, compared with other adsorbents. The acid-washing process can strongly enhance the adsorption capacity for A. oryzae mycelia. However, acid-washing does not alter the metal adsorption capacity of R. oryzae mycelia. The result indicates that acid-washing is not only a pretreatment step but also a regeneration step in the heavy metal removal process. These dual purposes, therefore, give the acid-washing biomass an indication of successful prospect. Cultivating A. oryzae in pellet form is an effective means of mycelium immobilization. The method established in this study provides the high-yield, uniformly-sized particles (2–3 mm in diameter), which are effective in solid-liquid separation. This pellet column can completely remove metals before breakpoint. After the breakpoint, a significant amount of Cu(II) removal over a long period has been observed. This is thought to be the result of intracellular uptake.

Photocatalyzed oxidation pathways of 2,4-dichlorophenol by CdS in basic and acidic aqueous solutions

Photocatalytic oxidation of 2,4-dichlorophenol (2,4-DCP) by CdS in the presence and absence of thioacetamide as a function of pH has been studied. Based upon the trends in the appearance of different intermediates such as benzene and hydroxylated biphenyl, it is proposed that 2,4-DCP oxidation by positive holes (h+) is favored at pH less than 6, while oxidation by hydroxyl radicals (·OH) becomes dominant at pH greater than 6. This is evidenced by the intermediates detected at different pH values. In the absence of thioacetamide (TA), benzene was detected as the only intermediate at pH ranging from 4 to 5. When pH changed from 5 to 9, no benzene was detected. Highly hydroxylated intermediates, such as hydroxylated biphenyl, were the major intermediates. When TA was present, it may act as a “bridge” between, 2,4-DCP and CdS through its thiol functional group. Therefore, a shift of the optimal pH for 2,4-DCP oxidation from 5 to 9 was observed and no hydroxylated biphenyl were detected.

Proton competition in Cu(II) adsorption by fungal mycelia

Fungal mycelia have been shown to be effective adsorbents for the removal of heavy metals from dilute solutions. Copper ions and protons become adsorbed to the fungal surface through reaction with specific surface sites including amino-, phosphate, carboxyl- or hydroxo-groups. In this study, an alkalimetric/metal titration technique was used to evaluate the competitive adsorption between protons and Cu(II) ions onto acid-washed mycelia of Aspergillus oryzae and Rhizopus oryzae. A general macroscopic mechanism has been established to describe the specific adsorption of both protons and Cu(II) ions. Using a simple graphic or a non-linear least squares technique, the adsorption parameters; i.e. adsorption constant and maximum adsorption capacity, were evaluated. The results predicted by the two-site model with a double reciprocal plot or the Scatchard plot were compared by non-linear regression. In the presence of multiple classes of sites, better fit of the experimental data was obtained by treating the fungal surface as one which possesses two major functional groups, namely: (1) high affinity of monodentate binding, (2) low affinity of bidentate binding. Because hydrogen ions compete significantly with Cu(II) ions for adsorption sites, the competitive Langmuir isotherm so derived was characterized by three parameters: two-site Cu(II) affinity, two-site Cu(II) adsorption density and apparent proton affinity. This enabled differentiation of the Cu(II) adsorption from protons. Two intrinsic equilibrium constants for Cu(II) surface complexation were estimated. With this information, it was possible to predict the adsorption of Cu(II) over a wide range of metal loading conditions at various pH values.

Ozonation of activated carbon and its effects on the adsorption of VOCs exemplified by methylethylketone and benzene

Ozonation can modify the surface property of an activated carbon such as specific surface area, pore volume, and functional group. Results indicate that ozonation can increase the specific surface area of an activated carbon from 783+/-51 to 851+/-25 m2/g due in part to increasing micropores (those below 15 A). However, there is no change in macropore and mesopore upon ozonation. The amount of oxygen functional group (OFG) increases from 197+/-4 to 240+/-4 microeq/g, mostly in hydroxyl and carboxyl groups upon ozone treatment. These oxygen-containing functional groups are stable in the temperature range 30-250 degrees C, but begin to decompose when temperature increases beyond 300 and 350 degrees C. When the temperature reaches 1200 degrees C, all OFGs virtually disappear. The effect of ozone treatment on the adsorption of volatile organic carbon (VOC) was exemplified by methylethylketone (MEK) and benzene. The adsorption density of MEK and benzene by ozone treated activated carbon (AC(O3)) are greater than that by the untreated (AC), with MEK being more adsorbable than benzene. Results of factorial analysis indicate that physical characteristics, namely, micropore, BET surface area, pore diameter (PD), micropore volume (MV) play an important role on benzene and MEK adsorption.

The removal of substituted phenols by a photocatalytic oxidation process with cadmium sulfide

Abstract The visible light photo-irradiation of aqueous cadmium sulfide produces conduction band holes that oxidize organic compounds. The oxidation rates of some substituted phenols depend upon the type of substituent as well as the degree of substitution. These rates do not correspond with those expected for electrophilic oxidation. Evidence for the rate dependence on the degree of phenol adsorption is presented which includes a correlation with the octanol-water partition coefficient as well as pH-surface charge dependencies.

The effect of chlorine position of chlorinated phenols on their dechlorination kinetics by Fenton's reagent

Chlorine position of chlorophenol isomers has a significant effect on the dechlorination kinetics of monochlorophenols, dichlorophenols, and trichlorophenols during Fenton oxidation. The effects have been evaluated by the rate constants of the dechlorination kinetic model developed in this study. It is found that the dechlorination rate of 3-CP is faster than that of 4-CP, which is faster than that of 2-CP. Since OH and Cl groups on the aromatic ring are ortho and para directors, the directory effect of OH and Cl groups enhances the dechlorination kinetics of 2-CP due to acceleration of the hydroxylation of 2-CP. Therefore, the dechlorination kinetics increases accordingly. For trichlorophenols (TCP), steric hindrance plays an important role during their dechlorination process. Specifically, the closer the chlorine atoms locate with each other on the aromatic ring, the more difficult the dechlorination processes will be. The dechlorination kinetics of dichlorophenols seems to be affected by both directory effect of OH and Cl groups and the effect of steric hindrance of chlorine atoms. The directory effect of OH and Cl groups on trichlorophenols decreases since the chlorine atom occupied the positions which are the most favorable for hydroxyl radical attack.