Application of tree barks to the removal of arsenic(V) and selenium(IV) species from coal fly ash leachates

Coal fly ash is generated during the combustion of coal for energy production in thermal power plants. Approximately more than 50% worldwide of all coal fly ashes is either stored in stockpiles, or disposed in ash landfills or lagoons. Fly ash is recognised as an environmental contaminants because of its high concentration of trace elements, such as As and Se, and the potential for leaching these elements into the wider environment. Several methods, excluding biosorption have already been reported for elemental removal from fly ash. Biosorbents, which refer to a group of compounds derived from the inactive, dead or microbial biomass, have the capacity to bind and potential utility in removing trace elements from wastewater systems. Tree bark as non-living biomass materials, contain proteins, carbohydrates and phenolic compounds, which can provide a wide variety of ion exchange sites. Additionally, tree barks as an environmentally friendly low cost biosorbents are readilyavailable, renewable resource with a large surface area, andable to potentially regenerateformetal recovery.In this study, the feasibilityof using the outer layer of selected tree barks as biosorbents for removing the most abundantarsenic and selenium speciation, arsenic(V) and selenium(IV), from fly ash leachate was investigated, and the results reveal that the selected tree barks are well effective for specified biosorption. In the beginning, three class F fly ash samples, including two acidic and one alkaline samples, have been assessed to characterise their physical(pH, moisture%, Brunauer-Emmett-Teller (BET)surface area and percentage of loss on ignition (LOI%)), chemical(major and minor elements)and morphological properties. Additionally, we have performed in this study a selection of leaching experiments under variableconditions (pH: 4, 7 and 11, solid: liquid ratios of 1:3.5 and 1:10, and contact time1 and 24 h) in order to determine the amount of As(V) and iiiSe(IV) present in fly ash leachates. Acidic fly ashes were found to release nearly 10% of As, twice the corresponding level in the alkaline fly ash leachate, whilst more than 50% of Se was removable from alkaline fly ash, nearly 10-fold the level in acidic fly ash leachate. Leaching experiments determined that fly ashes would tend to maintain its natural pH level regardless of the initial pH of the solutions used in leaching test. However,the initial pH exhibiteda major but variable effect on As(V) and Se(IV) mobility. Themobility of both As(V) and Se(IV) at a lower solid-to-liquid ratio of 1: 10 was found to be reduced by half in some cases. Nonetheless, during the early stage of leaching, As(V) and Se(IV) were quickly removed from enriched fine particles with great surface areas, whilst the mobility of these elemental species decreased with time, which may be due to sorption and/or co-precipitation of elements 'back into' the solid phase.Following the above study, a series of adsorption experiments was conducted on selected tree barks in order to assess the effects of bark-type, pH, contact time, biosorption dosage, and initial As(V) and Se(IV) concentration on adsorption process. In this study, barks of Eucalyptus deanei(Ed) and Melaleuca quinquenervia(Mq)have been applied as potential biosorbents for both As(V) and Se(IV) removal. The sorption of these elementalsspecies on bark was found to be highly dependent on solution pH.With increases in pH, sorption of As(V) and Se(IV) followed several increasing and decreasing trends which may be due to changes of an element-bark bond, which mayexist as strong inner-sphere covalent bonds and weak outer-sphere hydrogen bonds at different pH conditions. Specifically, the maximum sorption of As(V) occurred at pH 5 at Eucalyptus deaneibark(47.7%),and at pH 4 at Melaleuca quinquenerviabark(56.8%), while Se(IV) was sorbed mostly at pH6at Eucalyptus deaneibark (85.8%), and at pH 5atMelaleuca quinquenervia bark(84.5%). The sorption of both As(V) and Se(IV) was then determined to increase as a function of bark dosage and contact time, but decreased with a higher initial As(V) and Se(IV)concentration. The data for equilibrium sorption showed good fit to the Sips model for As(V) sorption, whilst the Langmuirand the Sipsisotherm models ivshowed better fit comparedto the Freundlich model for Se(IV)sorption.Accordingly, we have proposed here a sorption model consisting of a monolayer on a homogeneous bark surfacein higher element concentration,whilst in solutions with lower element concentration a multilayer sorption on the heterogeneous surface of bark may have occurred. A pseudo-second-order kinetic model was found to correlate most strongly with the experimental data for As(V) and Se(IV) sorption from aqueous solutions. Therefore, surface sorption is crucial in the As and Sesorption process. Desorption experiments indicated that sorption of As(V) and Se(IV) might follow an ion-exchange and strong physical-chemical sorption, which led to low percentage desorption of both elements.Finally, theremoval of As(V) and Se(IV) from the acidic andalkaline fly ash leachates wasdemonstrated using tree bark species of Eucalyptus deaneiand Melaleuca quinquenervia under optimised conditions of the biosorption process. It wasfound that approximately 69% to 100% As(V) can be removed from fly ash leachate in using Ed, and from 86% to 100% in using Mq. Moreover, 65% to 100% Se(IV) wereremoved from selected fly ash leachate using Edbark, and from 63% to 85% in using Mqbark.Overall, the present study has demonstrated that both the Eucalyptus deaneibark and Melaleuca quinquenervia bark are effective sorbents for As(V) and Se(IV) removal from fly ash leachates.