## Novel Adsorbent-Reactants for Treatment of Ash and

Scrubber Pond Effluents

Dr. Bill Batchelor Texas A&M University Zachry Department of Civil Engineering

3136 TAMU College Station, Texas 77843 979-845-1304 (voice) 979-862-1542 (fax)

bill-batchelor@tamu.edu

Grant No: DE-FG26-06NT42731 February 1, 2006 to January 31, 2009

Objectives : The overall goal of this project is to evaluate the ability of novel adsorbent/reactants to remove arsenic, selenium and mercury from ash and scrubber pond effluents while producing stable residuals for ultimate disposal. The adso rbent/reactants to be ev aluated are synthesized iron sulfide (FeS ) and pyrite (FeS 2 ). These compounds have the ability to remove arsenic, selenium and mercury from solution as well as to react with them to produce solid phases that are stable when disposed in landfills. Theref ore, removal of these co mpounds from wastewaters will be less likely to result in contam ination of soils and groundwaters.

Accomplishments: Methods for synthesizing pyrite an d iron sulfide were developed and analysis by TEM showed that they have particle sizes of 100 – 1000 nm and < 100 nm, respectively. Results of XRD an alysis confirmed that pyrite cr ystals were being produced by the synthesis method employed and that the form of iron sulfide being produ ced was Mackinawite.

Experiments have been conducted to describe removal of arseni c (As(III) and As(V)), selenium (Se(IV) and Se(VI)) and mercury (Hg) by iron sulfide (FeS) and pyrite (FeS 2 ). Initial kinetic experiments were conducted to determine appropriate times for conducting removal experiments. Batch removal experiments were conducted to eval uate the effects of c oncentration of target compound, pH and concentration of sulfate on rem ovals. These experiments were conducted in a way that is similar to that for systems in whic h adsorption equilibrium is believed to occur. The concentrations of target compound removed to the surface of the solid were calculated and expressed as the amount removed per mass of solid. The relationship of the solid-phase

concentration and the concentratio n of the target compound in the liquid phase was evaluated. In some cases, equations were used to model th is relationship and equation parameters were determined by least squares regressions. The mode ls that were used to correlate data were

models that are often used to correlate data obtained in e quilibrium adsorption experiments (Langmuir, Freundlich, BET). However, it is no t believed that the experiments conducted were at equilibrium, nor is it believed that adsorp tion was the only important process affecting the behavior of the target compounds. Results of these experiments indicate that the target compounds are initially removed by a mechanism th at can be considered to be adsorption, but then they react with the surfaces of the solids. The surface reac tions may not be at equilibrium when the experiment is completed.

Behavior of the target com pounds varied depending on which ad sorbent-reactant was used.Mercury was the target compound that was rem oved most strongly. The amounts of mercury removed were sometimes greater than the amount of adsorbent-reactant . Release of iron was small, so simple replacement of mercury for ir on would not explain obse rved results. Surface reactions are probably forming mi xed solid phases containing mercury, iron, sulfur and an anion (hydroxide, chloride, sulfate) removed from solu tion to balance the charge of mercury. The anion may be removed from solution along with mercury in the form of a mercury-anion complex. Sulfate was observed to have small effects on removal of mercury by both adsorbentreactants. Removal of both oxida tion states of arsenic by pyrite was observed to be affected by sulfate to small degree. An optimum pH for removal of As (V) was observed in the range between pH 8 to pH 9, while rem oval of As(III) continued to increase as pH increased. Removal of arsenic by iron sulfide showed more eviden ce of surface reactions. Removal of As(V) was improved at lower pH, while pH had little eff ect on removal of As(III) . Sulfate showed only small effects on removal of arsenic by iron sulfide. Removal of selenium in both oxidation states

showed some evidence of surface reaction on pyrite and iron sulfid e. Sulfate showed moderate

to small effects on removal of both forms of selenium.

The batch removal experiments indicate that arse nic, selenium and mercury can be removed by pyrite and iron sulfide. Generally low levels of inhibition by sulfate i ndicate that the process could be applied to waters high in sulfate, su ch as found in ash and scrubber ponds. In many cases, the target compounds appear to be remove d by pyrite or iron sulfid e and then react on the surface. This is shown by slow removal kinetics and increased removal at higher concentrations. These reactions are expected to produce lower sol ubility solid phases that will result in residuals that can be disposed with less likelihood of release.

Future Work: Experiments will be conducted to further ev aluate surface reactions of selenium, mercury and arsenic on pyrite and iron sulfide by investigating the leachability of the target compounds.

Publications/Awards/Students: No publications or awards have yet resulted from this work. Two graduate students have b een supported by this project – Mr. Dong Suk Han and Ms EunJung Kim.