Wayne E. Marshall

Production of granular activated carbons from select agricultural by-products and evaluation of their physical, chemical and adsorption properties.

Representative samples of soft, low density, group 1 (rice straw, rice hulls, sugarcane bagasse) and hard, high density, group 2 agricultural by-products (pecan shells) were converted into granular activated carbons (GACs). GACs were produced from group 1 and 2 materials by physical activation or from group 2 materials by chemical activation. Carbons were evaluated for their physical (hardness, bulk density), chemical (ash, conductivity, pH), surface (total surface area), and adsorption properties (molasses color removal, sugar decolorization) and compared with two commercial reference carbons. The results show that the type of by-product, binder, and activation method determine the properties of GACs. Regardless of the binder, sugarcane bagasse showed a better potential than rice straw or rice hulls as precursor of GACs with the desirable properties of a sugar decolorizing carbon. Pecan shells produced GACs that were closest to the reference carbons in terms of all the properties investigated.

Removal of selected metal ions from aqueous solution using modified corncobs

The objective of this study was to convert corncobs to metal ion adsorbents for wastewater treatment. Ground corncobs were modified with either 0.6 M citric acid (CA) or 1.0 M phosphoric acid (PA) to help improve their natural adsorption capacity. The effect of a combination of wash and modification treatment was tested for corncob adsorption efficiency with five different metal ions (cadmium, copper, lead, nickel, zinc) individually or in a mixed solution containing each metal at a 20 mM concentration. Results were compared to those of commercial resins Amberlite IRC-718, Amberlite 200, Duolite GT-73 and carboxymethylcellulose (CMC). Modified corncobs showed the same adsorption efficiency as Duolite GT-73 for cadmium, copper, nickel and zinc ions and had greater adsorption than CMC for nickel and zinc ions. For mixed metals, the modified corncobs exhibited the same adsorption efficiency as Duolite GT-73 for cadmium and copper ions and the same or higher adsorption than Amberlite IRC-718 for lead ions. Adsorption capacities of modified samples were compared to those of Amberlite IRC-718, Amberlite 200 and Duolite GT-73. Commercial resins generally had higher adsorption capacities than modified corncobs. However, the adsorption capacity of modified corncobs for copper and lead ions was equivalent to Duolite GT-73, but was lower than for Amberlite IRC-718 or Amberlite 200. Depending on the specific metal ion and the presence or absence of other metal ions, chemically modified corncobs were at least equivalent in adsorption properties to all of the commercial cation exchange resins examined in this study.

Freundlich adsorption isotherms of agricultural by-product-based powdered activated carbons in a geosmin-water system.

The present study was designed to model the adsorption of geosmin from water under laboratory conditions using the Freundlich isotherm model. This model was used to compare the efficiency of sugarcane bagasse and pecan shell-based powdered activated carbon to the efficiency of a coal-based commercial activated carbon (Calgon Filtrasorb 400). When data were generated from Freundlich isotherms, Calgon Filtrasorb 400 had greater geosmin adsorption at all geosmin concentrations studied than the laboratory produced steam-activated pecan shell carbon, steam-activated bagasse carbon, and the CO2-activated pecan shell carbon. At geosmin concentrations < 0.07 microg/l for the phosphoric acid-activated pecan shell carbon and below 0.08 microg/l for a commercially produced steam-activated pecan shell carbon obtained from Scientific Carbons, these two carbons had a higher calculated geosmin adsorption than Filtrasorb 400. While the commercial carbon was more efficient than some laboratory prepared carbons at most geosmin concentrations, the results indicate that when the amount of geosmin was below the threshold level of human taste (about 0.10 microg/l), the phosphoric acid-activated pecan shell carbon and the Scientific Carbons sample were more efficient than Filtrasorb 400 at geosmin removal.

Surface functional groups on acid-activated nutshell carbons

Nutshells from agriculturally important nut crops (almond, black walnut, English walnut, macadamia nut and pecan) were converted to granular activated carbon using phosphoric acid activation in nitrogen or air. Surface functional groups (carbonyl, phenols, lactones, carboxyl) were quantified by titration with bases of different ionization potential. The degree of copper uptake was correlated with the presence of various functional groups on these carbons. The results indicate that acid-activation strategies which provide the greatest contact of nutshells with air during carbonization and activation also have the greatest quantity of surface functional groups and the highest copper uptake. Also, the type of nutshell used for activated carbon production has little effect on the types of surface functional groups produced and subsequent copper adsorption. This study demonstrates that a judicious choice of activation strategy for nutshell precursors can produce activated carbons with numerous surface functional groups and high copper adsorption activity.

Agricultural by-products as granular activated carbons for adsorbing dissolved metals and organics

Surplus, low value agricultural by-products can be made into granular activated carbons (GACs) which are used in environmental remediation. This study characterized and evaluated GACs, made from these feedstocks, as effective removers of organics and metals from water. The by-products included soft lignocellulosics such as rice straw, soybean hull, sugarcane bagasse, peanut shell, and harder materials such as pecan and walnut shells. The softer materials were combined with a binder, molasses, to produce briquettes and pellets. The precursors were CO 2 - or steam-activated, and subsequent treatments included oxidation to enhance metal adsorption. Many of the GACs had acceptable physical GAC attributes, such as durability, for commercial usage. GACs made from pecan and walnut shells adsorbed higher levels of benzene, toluene, methanol, acetonitrile, acetone, and 1,4-dioxane from an aqueous mixture than commercial GACs. Neither CO 2 nor steam activation was particularly advantageous in enhancing metal adsorption. Oxidation using O 2 -N 2 gas increased metal adsorption while (NH 4 )S 2 O 8 solution did not. In a copper solution, oxidized GACs made from soybean hull had three to four times the Cu(II) adsorption capacity of metal-adsorbing, commercial GACs. Oxidized GACs made from soybean hull, sugarcane bagasse, peanut shell, and rice straw adsorbed from a mixture higher amounts of Pb(II), Cu(II), Ni(II), Cd(II) and Zn(II) than any commercial GACs. Commercial GACs adsorbed only Pb(II), Cu(II) and Cd(II). The GACs made from the agricultural by-products have considerable potential for adsorption of organics and metals of environmental concern.

Enhanced metal adsorption by soybean hulls modified with citric acid

Abstract A method was developed to enhance metal ion adsorption of soybean hulls for wastewater treatment using copper ion (Cu2+) as a typical metal ion. Hulls, extracted with 0.1 N NaOH, were modified with different citric acid (CA) concentrations (0.1-1.2 M) at 120°C for 90 min. CA-modified hulls had adsorption capacities for Cu2+ from 0.68 to 2.44 mmoles/g, which was much higher than for unmodified hulls (0.39 mmoles/g). The total negative charge for these hulls also increased with increasing CA concentration and was about twice the copper ion adsorption capacity at all CA concentrations. The need for NaOH (base) extraction (BE) before CA modification was examined. CA-modified, non-extracted (NE) and CA-modified, BE hulls were compared for adsorption kinetics and adsorption capacity. Base extraction resulted in modified hulls with faster adsorption kinetics and slightly lower adsorption capacity for copper ion than NE hulls. For BE, CA-modified hulls, increasing the temperature from 25°C to 60°C appeared to have no effect on the rate of copper ion removal from solution. CA modification of soybean hulls greatly enhanced metal ion removal and resulted in a product with possible commercial potential for metal ion remediation.

Acid-activated carbons from almond shells: physical, chemical and adsorptive properties and estimated cost of production.

Abstract A series of phosphoric-acid activated carbons were made from almond shells using six different activation or activation/oxidation methods. The carbons were compared to each other and to two commercial carbons in an effort to ascertain the relative value of the carbons in terms of yield, surface area, attrition, surface functional groups, organic uptake, metal uptake, as well as estimated cost of production. Of the six methods investigated, the method that produced the best overall performing almond shell carbon and least expensive carbon in terms of production cost was the “Air-Activation” method. This method involved the simultaneous activation and oxidation of almond shells under an air atmosphere.

Granular activated carbons from nutshells for the uptake of metals and organic compounds

Abstract Almond and pecan shells were chosen as hard, lignocellulosic precursors for the production of granular activated carbons (GACs) in order to create carbons for the adsorption of both organic compounds and metals. They were activated either chemically, with H 3 PO 4 , or physically, with CO 2 , under a variety of conditions. Following activation, a portion of the GACs were oxidized with air. The acid-activated samples had higher BET surface areas and greater product yields than the CO 2 -activated carbons. Unoxidized, CO 2 -activated carbons generally sequestered more Cu 2+ from solution than the unoxidized acid-activated GACs, when evaluated in batch assays at pH 4.8. Oxidative treatment, however, improved Cu 2+ adsorption in both types of carbon to levels significantly greater than comparable commercial carbons. Nutshell-based carbons were also examined for their ability to adsorb a variety of low molecular weight organic compounds with differing polarities. For CO 2 -activated pecan shell carbons there was a distinct increase in organic uptake that was usually not altered by oxidation. Both acid- and CO 2 -activated pecan shell carbons took up similar amounts of the non-polar benzene and toluene, but the CO 2 -activated carbons took up more polar compounds. There were several GACs that outperformed commercial carbons in their ability to adsorb significant quantities of Cu 2+ or organics on the same carbon. Our data show that nutshells provide a plentiful and inexpensive precursor for the production of GACs which may be competitive with commercial carbons in wastewater or potable water treatment.

Potential of agricultural by-product-based activated carbons for use in raw sugar decolourisation

The physical (bulk density and hardness) and chemical (pH and mineral content) characteristics of granular activated carbons (GACs) made from rice straw, pecan shells and hulls of soybean and rice were determined. The adsorption properties (iodine test, molasses test and raw sugar decolourisation efficiency) of these by-product-based carbons were also evaluated. A commercial decolourising carbon (Calgon CPG LF) was used as a comparison. Pecan-based carbons had bulk densities and hardness numbers higher than Calgon CPG LF. They also showed a mineral content and pH similar to or lower than the commercial carbon. The adsorption properties of the pecan-based carbons were, however, about 40% below that of the reference carbon. In contrast, rice (hull and straw)-based carbons exhibited good adsorption properties similar to the reference carbon in both iodine test and molasses test, but their ash content and their inherent pHs were higher than Calgon CPG LF. Both types of rice-based carbons were also more friable and had lower bulk densities than the commercial carbon. However, rice-hull-based carbon was statistically as effective as Calgon CPG-LF in decolourising raw sugar solutions at the 5% confidence level. Of the by-product-based carbons evaluated, pecan-shell-based GACs would be most suitable as raw sugar decolourisers if their adsorption efficiencies could be improved.

Adsorption of volatile organic compounds by pecan shell-and almond shell-based granular activated carbons

The objective of this research was to determine the effectiveness of using pecan and almond shell-based granular activated carbons (GACs) in the adsorption of volatile organic compounds (VOCs) of health concern and known toxic compounds (such as bromo-dichloromethane, benzene, carbon tetrachloride, 1,1,1-trichloromethane, chloroform, and 1,1-dichloromethane) compared to the adsorption efficiency of commercially used carbons (such as Filtrasorb 200, Calgon GRC-20, and Waterlinks 206C AW) in simulated test medium. The pecan shell-based GACs were activated using steam, carbon dioxide or phosphoric acid. An almond shell-based GAC was activated with phosphoric acid. Our results indicated that steam- or carbon dioxide-activated pecan shell carbons were superior in total VOC adsorption to phosphoric acid-activated pecan shell or almond shell carbons, inferring that the method of activation selected for the preparation of activated carbons affected the adsorption of VOCs and hence are factors to be considered in any adsorption process. The steam-activated, pecan shell carbon adsorbed more total VOCs than the other experimental carbons and had an adsorption profile similar to the two coconut shell-based commercial carbons, but had greater adsorption than the coal-based commercial carbon. All the carbons studied adsorbed benzene more effectively than the other organics. Pecan shell, steam-activated and acid-activated GACs showed higher adsorption of 1,1,1-trichloroethane than the other carbons studied. Multivariate analysis was conducted to group experimental carbons and commercial carbons based on their physical, chemical, and adsorptive properties. The results of the analysis conclude that steam-activated and acid-activated pecan shell carbons clustered together with coal-based and coconut shell-based commercial carbons, thus inferring that these experimental carbons could potentially be used as alternative sources for VOC adsorption in an aqueous environment.