Surface modification of electric arc furnace slag through acid treatment for removal of methylene blue from aqueous solution

In line with the rapid growth of steel manufacturing industries, the amount of slag generated has increased tremendously. Currently, slag is less utilized, and this industrial byproduct has been disposed of in a landfill or incineration. The idea of recycling slag into an adsorbent for industrial wastewater treatment is a sustainable approach that facilitates double benefits, prolonging the landfill's lifespan and converting the waste material into a useful application. Thus, this work investigated the ability of untreated and treated electric arc furnace (EAF) slag to remove methylene blue dye from the aqueous solution. The batch adsorption and fixed-bed column adsorption were performed using varying experimental parameters to identify the optimal adsorbate uptake. The EAF slag was chemically treated by adapting the wet oxidation process using hydrochloric acid solution and ammonium chloride solution. This treatment aims to further improve the surface properties and characteristics of the adsorbent to specifically target the uptake of methylene blue dye through adsorption. Accordingly, the treatment with hydrochloric acid (HCl) solution has greatly increased the BET surface area up to 98.24 m2 /g, introducing the haloalkane functional group and significantly leaching calcium from the surface. The treatment with ammonium chloride (NH4Cl) solution has tremendously increased the pore size to 108.007 nm. The adsorption process utilizing the untreated EAF slag reportedly depended on pH and initial dye concentration. In contrast, the treated EAF slags were discovered to be dependent on pH, adsorbent dosage and initial dye concentration. The maximum adsorption capacity demonstrated by the untreated EAF slag was 6.5 mg/g. As for the HCl treated and NH4Cl treated EAF slag, the maximum adsorption capacities were 13 mg/g and 14.35 mg/g, respectively. At the same time, fixed-bed column adsorption utilizing the untreated and treated EAF slag was influenced by the bed height, initial dye concentration and flow rate. Notably, lower bed height has demonstrated extended breakthrough and exhaustion time. Meanwhile, higher initial dye concentration and higher influent flow rate are attributed to faster breakthroughs and exhaustion times. The adsorption isotherm has demonstrated the highest linear regression value (R2 ) by the Langmuir model graphical plot, signifying monolayer adsorption. In addition, the kinetic studies revealed that the untreated and HCl treated EAF slag samples were well fitted into the pseudo-second-order model and the NH4Cl treated EAF slag was reported to best fit the Elovich model, signifying chemisorption. The kinetic studies performed on the breakthrough curves revealed that all the EAF slag samples were well-suited to the Thomas, Yoon-Nelson, and Adams- Bohart models, indicated by the lower sum of squared error (SSE) values. The economic analysis has indicated that the adsorption process that utilized the treated EAF slags requires additional cost compared to the untreated EAF slag due to the cost of materials used for the adsorbent treatment. However, the energy usage in the adsorption process was much lower compared to the cost required for the adsorbent treatment. Based on the experimental findings, the treated EAF slags are a promising adsorbent with a higher adsorption capacity in removing methylene blue dye. Although additional cost is required to prepare the treated EAF slags, no cost is imposed for the material's purchase as it is an unutilized industrial waste material that is present in abundance. The higher adsorbate uptake made the adsorbent attractive for usage. Conversely, this study's energy consumption cost is much lower than the energy consumption required in synthesizing the carbonaceous-based adsorbent.

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