Jute (Corchorus olitorius) stick charcoal: a potential bioadsorbent for the removal of Cr(VI) from an aqueous solution

Industrial effluents emit a variety of contaminants, with heavy metals being one of the most serious hazards to the environment. Heavy metals have deadly impacts on flora and fauna when they are above the acceptable limit (Saranya et al. 2017). One of the most harmful heavy metals is chromium, which exists in the effluent from industries that use chromium, such as industries involved in leather tanning, electroplating, textile production, and chromium-based product manufacturing (Ghosh et al. 2018). When chromium exceeds the allowed limit due to its half-filled electrical form, it is hazardous to the ecology. In aqueous solutions, the two stable forms of Cr are Cr(III) and Cr(VI). (Saranya et al. 2018). Cr(VI) is around 300 times more toxic than Cr(III) and because it has high water solubility, it can transport long distances and is highly bioavailable (Ghosh et al. 2018). The kidneys, skeletal system, hematological system, and central nervous system in the human body are affected by chromium toxicity (Kaduková & Vircíková 2005). Chromium levels in drinking water should not be more than 0.05 mg/L (WHO 1993). Though industries have treated effluents containing harmful compounds such as Cr(VI), if they exceed permissible levels, they endanger soil, water, air, and ecosystems (Saranya et al. 2018). Different methods were used to remove this pollutant from water including ion exchange precipitation, electrocoagulation, adsorption, and reverse osmosis (Khalifa et al. 2019) Among heavy metal removal technologies, by far the most versatile and extensively utilized technique is adsorption (Ghosh et al. 2018). Over several decades, researchers have employed different biomasses like Date palm empty fruit bunch, Coconut fibers, Datura stramonium fruit, Sargassum horneri (Torres 2020), rice husk, and rice husk ash (Ghosh et al. 2018) for Cr(VI) removal from aqueous solutions using biosorbents. On the contrary, activated carbon or modified activated carbon is considered a possible adsorbent for pollutant removal and is largely used due to its wide surface area, significant porous space, high removal efficiency, different functional groups on the adsorbent surface, fast adsorption kinetics, chemical characteristics, and distinctiveness of regeneration (Ghosh et al. 2020). Thus, special high lingo-cellulosic agro-residues such as jute stick charcoal (JSC) may offer Cr(VI) removal from contaminated water using a readily available, no/low-cost raw material. Jute (Corchorus olitorius) is mostly grown in Asian nations such as Bangladesh, India, Bhutan, China, Nepal, and Thailand, and is commonly referred to as the ‘golden fiber’ (Ray & Ghosh 2018). Jute stick manufacturing on a global scale is predicted to be over 6 million tons annually (Chakraborty et al. 2020). Bangladesh is now the world's second-biggest producer of jute. Our nation contributes more than 70%, making it the world's largest exporter of jute fiber (Hossain & Abdulla 2015). Only 4–5 months are needed for jute fiber to grow and create such a large number of ligno-cellulosic sticks (hemicellulose ∼30%, cellulose ∼41%, lignin ∼24%) which composition-wise is equal to woods grown over many years. The jute stick is not good for mulching nor as a feed (Ghosh et al. 2019) and as a result, is widely utilized as a source of fuel in rural areas. In this study, JSC was chosen as an adsorbent due to its availability, cost-effectiveness, and eco-friendly criteria. To the best of our knowledge, there is no study on the removal of Cr(VI) by JSC. Therefore, the objectives of this study are to investigate the applicability of JSC for Cr(VI) removal from aqueous solutions by influencing different experimental parameters like contact time, pH, adsorbent dose, and initial Cr(VI) concentration. Finally, the adsorption behaviors of JSC for Cr(VI) were studied by different kinetics and isotherm models.

A stock solution of 1,000 mg/L potassium dichromate (K₂Cr₂O₇) was made by dissolving potassium dichromate (Grade: ACS reagent, Merck, Germany) in distilled water. A portable digital pH (model pH⁵⁶, Milwaukce Instruments, Inc., USA) meter was used to determine the pH of the solution. The concentration of the Cr(VI) ion in water was determined using a spectrophotometer (HACH DR, 3900; method 8023) employing the 1,5-diphenylcarbohydrazide technique and a single dry powder formulation termed ChromaVer^® 3 chromium reagent for Cr(VI). We determined the total dissolved solids (TDS), electrical conductivity (EC), and salinity using a portable conductivity/TDS meter (sensION ⁺EC5, HACH). The applicability of JSC for Cr(VI) removal from tannery effluent was determined using a sample of tannery effluent taken from a tannery in Hazaribagh Tannery Area, Dhaka, Bangladesh.

Jute sticks were gathered in the village of Jashore, Bangladesh, then chopped into little pieces (5 inches long). After that, the surface was cleaned with regular tap water and then again with distilled water before being dried at 70 °C in a hot air oven for 48 h to remove any bound substances and pollutants. The dried jute stick was then carbonized for 30 min at 300 °C in a muffle furnace (SXT-10, Shanghai Shuli Instrument and Meters Co., Ltd, China). The carbonaceous jute stick was then cooled, pulverized in a mortar, and sieved to obtain particles with a diameter of 0.5–1.0 mm. Finally, this adsorbent was ready for usage and was packaged in sealed borosilicate glass vials for further investigation. JSC FTIR (Fourier transform infrared spectroscopy) spectra were noted (before Cr(VI) adsorption) using an FTIR-4600 spectrophotometer (JASCO Corporation Ltd, Japan). A field emission scanning electron microscope was used to study the surface morphology of JSC (pre-Cr(VI) adsorption) (FE-SEM, Zeiss Sigma, Carl Zeiss, Germany).

Unfavorable adsorption is indicated by R_L > 1, linear adsorption is indicated by R_L = 1, favorable adsorption is indicated by 0 < R_L < 1, and irreversible adsorption is indicated by R_L = 0.

K_id is the intraparticle diffusion constant rate (mg/g/min^0.5), which may be calculated using the linear plot slope q_t vs. t^0.5, and C denotes the plot intercept, it denotes the boundary layer's effect. When the intercept is larger, the rate control step is influenced more by surface adsorption (Wu et al. 2005).

The concentration of hydrogen ions in liquids has an impact on the movement of other ions in an aqueous solution. The effect of pH on metal sorption has been demonstrated in many studies. The rate of Cr(VI) biosorption was primarily influenced by pH, and the study result is presented in Figure 3(b). When the pH of the solution increased from 2 to 8, the Cr(VI) removal by JSC decreased from 99.0 to 4.0%. The Cr(VI) maximum (99%) adsorption was detected at pH 2.0. Protonation causes the biosorbent to be positively charged at low pH levels. An electrostatic attraction occurs when the dichromate ion functions as the anion (Boddu et al. 2003). The adsorption sites are occupied by anionic species, for example, Cr₂O₇²⁻, CrO₄², HCrO₄⁻, and the reduction in adsorption above pH 4 might be described by the availability of chromium oxyanion and OH⁻ ions in the bulk (Dönmez & Aksu 2002). Al-Homaidan et al. (2018) observed a similar type of result.

Figure 4(b) shows that the removal percentage of Cr(VI) decreased (99–84%) with an increasing initial Cr(VI) concentration (10–100 mg/L). The effectiveness of adsorption is determined by two elements. First, when chromium ion concentrations are lower, it provides a positive force that accelerates the adsorption process, and when there are more chromium ions, there is more competition for binding sites in the biomass. Second, when chromium concentrations rise, the proportion of chromium removed decreases. This might be owing to the binding sites becoming saturated (Al-Homaidan et al. 2018). On the other hand, JSC adsorption capacity increased (1.24–10.52 mg/g) when the Cr(VI) concentration (10–100 mg/L) increased (Figure 4(b), due to available Cr(VI) ions in aqueous solutions.

JSC maximal adsorption capacity, according to the Langmuir isotherm, was 11.429 mg/g (Table 1). The Elovich isotherm's reduced adsorption capacity (q_m = 3.31 mg/g) and R² (0.974) values show that it is unsuitable for explaining the adsorption process of Cr(VI) onto JSC (Figure 5(c)). Table 2 shows JSC's capability for absorption compared to other adsorbents defined in the literature. It indicates that Cr(VI) has a modest maximum monolayer adsorption capacity on JSC and that it is a good alternative adsorbent for the cleaning of heavy metal Cr(VI) from an aqueous solution because of its ease of availability, cheap cost, no secondary contamination, and environmental friendliness.

So, it is ratified that the sharing or exchange of electrons among Cr(VI) and adsorbent (JSC) and the adsorption process was chemisorption (Bhattacharya & Sharma 2005). The Cr(VI) diffusion process onto JSC was investigated with the intraparticle diffusion model. Throughout the intraparticle diffusion process, metal ions are transferred from solution to the solid phase (Kaya et al. 2014). If the data indicate multi-linear plots, the process is divided into two or more parts. Figure 6(c) clearly shows three separate zones: the first linear section (phase I), the second linear part (phase II), and lastly the third linear part (phase III). The first linear portion may be responsible for the rapid use of the most readily accessible adsorbing sites onto the adsorbent surface (phase I). The slow diffusion of the adsorbate from the surface area into the interior pores is known as phase II. Finally, phase III is associated with very slow diffusion. Thus, the first component of Cr(VI) adsorption by JSC might be driven by Cr(VI) intraparticle transport exactly by surface diffusion and the latter portion by pore diffusion. On the other hand, the intraparticle diffusion model linear plot did not reach the start, which might be attributable to the difference in mass transfer rates between the start and end phases of adsorption. As a result, the fact that the straight lines from the origin diverge indicates that pore diffusion is not the individual rate-controlling mechanism as shown by the divergence of the straight lines from the origin.

Bhatnagar et al. (2010) found similar types of results. The intraparticle diffusion model's linear plot (Figure 6(c)) did not reach the start due to variations in mass transmission from the initial to final stages of the adsorption method, and the intercept (0.796–2.338) value was greater than 0.0, indicating that multiple processes (surface, pore, or firm diffusion) were involved through Cr(VI) adsorption on JSC and that they may have occurred simultaneously (Ghosh et al. 2020). The Plot of Boyd kinetic was examined with various contact times of adsorption data to determine the real step through the adsorption procedure. The Boyd plot did not reach the start in Figure 6(d), showing that film diffusion is a part of the adsorption process.

The findings of the current study indicate that the JSC may be effectively utilized as a biosorbent for Cr(VI) removal from aqueous solutions with various controlling parameters like contact time, pH, initial Cr(VI) concentration, and adsorbent dose. The maximum removal was found at pH 2. The Freundlich and Halsey isotherms had a better fit with the experimental data. The highest adsorption capacity of JSC for Cr(VI) removal was found as 11.429 mg/g. Kinetic studies represented that the adsorption process followed the pseudo-second-order kinetic model with multi-steps diffusion process for the adsorption of Cr(VI) on JSC. The FE-SEM and FTIR analyses confirmed that the surface of JSC contains more porous active sites and a variety of functional groups. As a result of its availability, cheap cost, adsorption capacity, and good kinetics, JSC has significant potential for practical use in the adsorptive Cr(VI) removal from an aqueous solution.

We would like to thank the Department of Environmental Science and Technology, Jashore University of Science and Technology, Bangladesh, for providing the necessary support. We also thank the Ministry of Science and Technology, Bangladesh, for the research grant (R&D) award.

The authors would like to thank the Ministry of Science and Technology, Bangladesh, for the research grant (R&D) award.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.