Faculty and Staff
Research of Dr. David Rockstraw, P.E.
Synthesis and Characterization of Activated Carbon from Lignocellulosic Raw Materials
What is Activated Carbon?
Activated carbon is a term used for a material that have been treated in some manner allowing the treated material to be able to remove toxic or problem chemicals from water. Activated carbon is produced from wood, coal, lignite, coconut shells, and peat. These materials are starting materials for 97% of the activated carbon produced. Pecan shells represent a material similar to coconut shells from which activated carbon with desirable properties can be produced. A novel method for synthesizing activated carbons has been developed and patented in the NMSU Department of Chemical Engineering. The carbon product manufactured by this method is referred to as PS276a.
Why Pecan Shells?
Pecan production in the United States iss around 3x108 pounds in 1997. About half of that mass is shell, thus over 1.5x108 pounds of shell waste was generated last year. The pecan shell currently has no commercial value, and most of the waste is placed in landfills. With seed funds provided by the Waste-management Education and Research Consortium (Department of Energy), a carbon material has been developed using pecan shells as the precursor. This carbon has been tested on a number of systems, each showing a high affinity for the carbon. The virtues of the carbon lie not only in its separation capabilities, but also in the novelty of its manufacture. A new, low-temperature, low-ash manufacture method has the potential to introduce a new form of carbon material to commercial markets for use in chemical separations.
Results and Benefits Expected of current research efforts:
Design an activated carbon with a high affinity for nitro- and nitramine-based organic molecules based on a technology developed at New Mexico State University by attaching surfactant molecules to the chemical functionalities associated with the carbon surface, then characterize the developed carbon using a combination of conventional and developed analytical methods and quantify the kinetics of the activation process based on fundamental functional group chemistries.
Design a carbon that will absorb high explosive chemicals in concentrations sufficient to be useful as a component in a bio-reactive barrier wall under development at LANL; develop an equilibrium model based on experimental data that predicts isotherm concentrations based on functional groups of the organic adsorbates; and develop a kinetic model of the manufacture process that allows prediction of carbon surface functionality based on the distribution of cellulose, hemicellulose, and lignin contained in an agricultural waste
Project Status:
This project has received numerous accolades, including the 1997 American Academy of Environmental Engineers' Research Grand Prize, the first time this award was made to an academic project. It has also been featured on CNN's Science and Technology Week.
This project is currently investigating use of the patented methods on other lignocellulosic raw materials, including the seeds of the chile pepper.
Synthesis of Potassium Ferrate and its Application to Environmental Systems
What is potassium ferrate?
Ferrate(VI) ion is a highly-oxidized form of iron (+6 oxidation state) and is considered by many to represent the next-generation of green oxidants. However, there are currently no commercial suppliers of ferrate(VI) salts as an economically attractive production method has not been identified, barring the use of chemistries using ferrate(VI) for industrial applications. The ferrate(VI) ion, FeO42-, is a tetrahedral ion that is isostructural with chromate. Redox potentials for ferrate(VI) have been estimated in both acidic and basic media:
REACTION 1: 3e- + 8 H+
+ FEO42- → FE3+ + 4 H2O
E° = 2.20 V
REACTION 2: 3e- +4 H2O + FEO42- → FE3++ 8 OH- E° = 0.72 V
Compared to the redox potential of other oxidants and disinfectants (chlorine, 1.358 V; hypochlorite, 1.482 V; perchlorate, 1.389 V; ozone, 2.076 V; hydrogen peroxide, 1.776V; permanganate, 1.679), ferrate(VI) is a significantly higher, yet it's by-products are benign.
Ferrate is further known to be a strong oxidant that can react with a variety of inorganic or organic reducing agents and substrates. Ferrate(VI) can act as an important and selective oxidant for synthetic organic studies and is capable of oxidizing/removing a variety of organic compounds from, and of destroying many contaminants in aqueous media.
Synthesis of ferrate(VI)
Ferrate(VI) production dates to 1702 with the work of Stahl, though it was not until the 1950s when x-ray analysis demonstrated that potassium ferrate was similar in structure and size to potassium chromate. Modern synthesis of ferrate(VI) include three methods: dry oxidation, wet oxidation, and electrolytic.
Electrolytic techniques have been described in both the open and patent literature, yet the art remains inefficient, which accounts for the absence of a commercial process to manufacture ferrate(VI) by one of these methods. Yield based on current is typically less than 5%, though recent advances claim a current yield of 20% based on solution concentration of potassium ferrate(VI) produced (not recovered yield). Current efficiencies of 70% have been reported using silver steel with a carbon content of 0.90%, though only a 12% efficiency was reported when carbon content was reduced to 0.08%. Clearly, electrolytic technology must mature before commercial implementation.
Chemical techniques have received attention recently as the significance of ferrate(VI) as an environmentally-friendly oxidant is learned. While a variety of techniques have been reported, most researchers use some form of alkaline oxidation involving the hypochlorite oxidation of Fe(III) in strong alkaline solution. Hypochlorite ion is produced by the disproportionation of chlorine in cold caustic. This method requires chlorine gas, large amounts of caustic soda and potassium hydroxide, and many strictly controlled steps and conditions. Finely divided ferric nitrate is added to the hypochlorite solution. Grinding the iron is necessary to promote complete dissolution of the ferric ion into the alkaline solution. Reagent purity must be ensured for maximized ferrate(VI) yield and purity. To minimize catalytic decomposition of ferrate during synthesis, membrane cell-grade aqueous KOH must be used. The chlorine gas must be "substantially pure" produced by chlor/alkali cells, and the source of ferric salt must be controlled. A complete prior-art review is contained in the Johnson patent. While this chemistry may be conducive to lab preparation, commercial production by this technique is unlikely.
The method of Johnson for production of ferrate ion involves the monoperoxosulfate oxidation of Fe3+ to FeO42-, which is subsequently precipitated as K2FeO4.
Reaction 3: 3 HSO5- + 3 Fe3+ + 5H2O → 2 FeO42- + 3 HSO4- + 10 H+
The procedure requires simple mixing of ferric sulfate and monoperoxosulfate followed by addition of KOH. The method is a low-cost, safe, and chemically simple route to potassium ferrate that circumvents the use of chlorine. Because the hypochlorite oxidant is eliminated, less KOH is necessary, resulting in decreased solution viscosity and decreased filtration time during the isolation. Monoperoxosulfate (manufactured by DuPont as Oxone) aids to circumvent filtration problems associated with hypochlorite.
Chemistries of ferrate(VI)
Chemistries of ferrate(VI) are receiving more attention in the literature of late as researchers come to understand the potential of this species as an environmentally-safe, alternative oxidizing agent. While recent literature is available on classifications of chemistries, a recent review of ferrate(VI) chemistry (Sharma, 2002) shows an emphasis on the application of ferrate(VI) to organic reaction systems. There are a number of researchers who have worked with ferrate(VI) in sulfur-containing systems, the focus of work in this lab at NMSU. Published studies include the oxidation of hydrogen sulfide, sulfonic acids, and various sulfur-containing organic and inorganic oxysulfur species.
The oxidation of Arsenic(III) by potassium ferrate(VI) was studied. About 80 % wt of As(III) in solution was oxidized. With decreasing EPA limits, the proposed technique will have potential application to As(III), increasing it's value to society.
To date, no known work has been published on the chemistry of ferrate(VI) with metal sulfides, except for that performed by Murshed, et al. as summarized in the paragraphs that follow. Use of ferrate in this manner has the potential to be a large commercial application.
A chemistry utilizing ferrate(VI) has been identified for the treatment of mine tailings that uses the ferrate ion (FeO42-) as an oxidizing agent, and mitigates the acid production potential of the tailing by rapidly oxidizing sulfides into sulfates, while enhancing extraction of metal species by liberating sulfide-bound metals.
This chemistry has been studied through use of a scanning electron microscope (SEM) and associated x-ray analysis (XRA) to quantify elemental concentrations in the solids, and an inductively-coupled plasma mass spectrometry to measure metal concentrations in the extract. Under conditions of this preliminary research, reactions occur in a mass transfer-limited regime with a rate on the same order as the reduction of ferrate by oxidation of water. In spite of this limitation, sulfide concentrations in the solid tailings were reduced by greater than an order of magnitude. An order-of-magnitude increase in metal liberation into the aqueous extract was also observed.
Sulfidic minerals appeared to have interacted with ferrate(VI) in a manner analogous to a geochemical oxidation of sulfide in which ferric(III) ion is the oxidant as in Reaction 4, The analogous oxidation by ferrate(VI) is shown in Reaction 5, and for the generic divalent metal M in Reaction 6.
Reaction 4: FeS2 + 14 Fe3+
+ 8 H2O → 15Fe2+ + 2SO42- +
16H+
Reaction 5: FeS2 + 5 FeO42-
+ 12 H2O → 6 Fe(OH)3 + 2 SO42-
+ 24 OH-
Reaction 6: MS + 2 FeO42-
+ 4H2O → M2+ + 2 Fe(OH)3 + SO42-
+ 2 OH-
In our work, the reaction between ferrate(VI) and tailings will take place in the solid-state in a ball mill reactor, in the reduced presence or absence of solvent. Side reactions leading to reduction of ferrate(VI) by solvent will thus be minimized. Upon reaction completion, solid materials will be washed from the milling reactor to separate the sulfides into the aqueous phase for treatment. Treated tailings solids will be filtered.
Potassium ferrate(VI) synthesis
Ferrate(VI) production by the method of Johnson uses concentrated KOH solution to precipitate the potassium salt of the ferrate(VI) ion. As KOH concentration increases, the viscosity of the solution also increases, affecting the mass-transfer characteristics of the system. As KOH concentration is decreased, mass-transfer is improved, but the effectiveness of precipitating ferrate(VI) salts with KOH is diminished. In these reactions, water serves as a solvent, a reactant in the primary ferrate(VI) chemistry, and as a reactant in the ferrate(VI) degradation chemistry.
Consequently, the kinetics and mass-transfer limitations of ferrate(VI) manufacture must be understood to scale-up a facility for commercial manufacture of this material by this chemistry. This stoichiometry and physical chemistry of this reaction system will be studied in a manner from which a complete accounting of the mole balance can be performed. In addition, relevant rate parameters will be obtained for the design and optimization of the unit operations necessary to produce scale quantities.
To carry out the production of potassium ferrate(VI) by the method of Johnson, ferric sulfate will be mixed with Oxone (monoperoxosulfate). The two solid materials will be blended to achieve as homogenous of a mixture as possible. Potassium hydroxide solution or potassium hydroxide will be added to the solids mixture while agitating and maintaining a constant temperature (between -10°C and 30°C).
This reacting solution becomes purple and is filtered through a coarse, fritted-glass filter. Precipitation of solid potassium ferrate is accomplished with the addition of saturated KOH, followed by chilling. After precipitation, the solid is filtered through a medium fritted glass filter and dried with an organic solvent (diethyl ether) to prevent the other species from being oxidized by the ferrate in the presence of water. The crystal product is dried under vacuum. The procedure can be performed in an hour.
In the second method to be applied to the production of ferrate(VI), reactants will be contacted in a ball mill at constant temperature. A ball mill is a container in which solid reactants are placed, along with a number of grinding balls. The container is continually tumbled, causing the solids to be agitated, contacted, and ground by the action of the balls. The device thus affects solid-solid contact. Grinding jars in a centrifugal ball mills move in a horizontal plane. Centrifugal forces generated by the motion propel the grinding balls against the inside wall of the jar, where they roll over the materials. Grinding is effected by impact and friction. To counter agglomeration and enhance homogenization, direction of rotation is periodically reversed.
Flash Pyrolysis of solutions to form metallic nanoparticles
What is flash pyrolysis?
Flash pyrolysis involves the introduction of an atomized spray of an aqueous solution of metal into a high temperature reactor in which the solvent is vaporzied and the metal is reduced. The resulting metallic particle is of the size of the metal content of the atomized droplet. Highly monodisperse nanoparticles of silver, ruthenium, and nickel have been manufactured in this manner, with average particle sizes controllable between 10 and 500 nm. A technique has been found by which nanoparticles of one metal can be produced, encapsulated by a second metal (see image).
Project Status
This project has been funded by LaSys, Inc., and through a series of SATOP grants. A proposal has been submitted to NSF-04043, NER: Synthesis of encapsulated binary metallic nanoparticles by flash pyrolysis. It is staffed by Ph.D. student Kalyana Pingali Charavarthy.