MODIFIED WALNUT SHELLS FOR THE REMOVAL OF MERCURY

Abstract

Mercury, a highly toxic heavy metal, poses a significant threat to both the environment and human health. Mercury pollution can negatively impact wildlife and infiltrate the food chain, ultimately affecting humans. Exposure to mercury can lead to brain impairment, developmental issues, and kidney failure. It is essential to enforce tougher restrictions and create cleaner technologies to reduce its discharge. In the design of an optimized activated carbon-based adsorbent, high BET surface area, high adsorption capacity, selectivity, mechanical and thermal stability, regeneration potential, environmental impact and economical cost are all key characteristics to the practical application of activated carbon in the adsorption process for mercury removal. Despite its many advantages, the synthesis of activated carbon encounters certain obstacles that impede its extensive and optimal application. While some challenges have been addressed in literature, such as replacing traditional sources that are non-renewable with sustainable and readily available alternative carbon precursors-which is paramount for long-term production. Other challenges such as the usage of toxic chemicals for the purpose of tailoring surface chemistry and pore size still require thorough investigation. In addition, low carbon yield during the production of activated carbon still requires detailed understanding and identification of an effective method to increase carbon yield while maintaining an eminent BET surface area. To address the problem concerning the usage of toxic chemicals for chemical activation, four environmentally friendly inorganic salts were selected for experimenting as chemical activating agents for the synthesis of activated carbon, which are potassium carbonate (K2CO3), potassium bicarbonate (KHCO3), sodium thiosulfate (Na2S2O3) and potassium sulfate (K2SO3). The activated carbon sample activated with K2CO3 (WSAC) exhibited the highest BET surface area of 1046.28 m2/g at an impregnation ratio of 1:2 (ratio of carbon precursor to activating agent) and activation temperature of 800 °C. The activated carbon sample activated with Na2S2O3 (STAC) generated the highest carbon yield around 12%. Poorly performed activating agents were discontinued from further investigations. In the aim of further enhancement, K2CO3 and Na2S2O were combined to synthesize a novel adsorbent (CAC) that could attain a high BET surface area while maintaining an acceptable carbon yield. The novel adsorbent has demonstrated a BET surface area of 2132.7 m2/g. The efficiency of those AC adsorbents was tested for the removal of mercury. CAC generated the highest maximum adsorption capacity (Langmuir) of 289 mg/g at room temperature, while the adsorption capacities of the other two were determined to be 214.1 and 164.4 mg/g at 35 °C respectively. WSAC and CAC reached equilibrium after 4 hours of contact time, meanwhile STAC took 6 hours. The experimental data pertaining to WSAC and CAC were adequately described by the Langmuir isotherm, while STAC’s experimental data were fitted into the Freundlich model. As for the kinetic modeling, all three were well described by the Pseudo-second-order model. The main mechanism of removal was established to be through complexation of mercury with oxygen and sulfure-containing functional groups and pore filling. All three adsorbents exhibited high affinity towards mercury. In addition, a regeneration study was conducted up to 5 cycles. This research introduced a green and novel adsorbent that is cost-effective for mercury removal.