Adsorption dynamics of dye onto crab shell chitosan/neem leaf composite

Dye-containing effluent from paint, leather, textile, pulp and paper industries is a serious ecological concern (Long-Fei et al. 2017; Shany & Giora 2018). Most dyes are toxic in water and possibly carcinogenic, and can stay in the environment for a long period (Mehdi et al. 2018; Wenjue et al. 2018). Such pollutants affect not only aquatic ecosystems, but also human health and the food chain. Methylene blue (C₁₆H₁₈CIN₃S) is a typical example of a toxic cationic dye commonly used to dye silk, cotton, and wool (Thakur et al. 2016; Miyah et al. 2018). For that reason, environmental protection agencies have listed methylene blue among the priority pollutants to be removed from water streams (Hameed & Rahman 2008; Jia & Lua 2008).

Several conventional methods are available in wastewater treatment for removing methylene blue, these includes ion exchange, precipitation, biodegradation, ozone treatment, membrane filtration, photocatalysis, coagulation, flocculation, oxidation and adsorption (Aysan et al. 2016; Rajasulochana & Preethy 2016; Luo et al. 2017; Wang et al. 2017). These methods often have high operating and maintenance costs, and special handling for hazardous by-products disposal (Tabari et al. 2012). Among them, adsorption is favoured because it is relatively cheap, simple and easy to operate, and effective in removing low concentration pollutants (Aysan et al. 2016; Lakshmi et al. 2016; Pathania et al. 2017; Tahir et al. 2017; Apurva et al. 2018).

The development of a natural, low-cost adsorbent for methylene blue has become increasingly attractive (Annaduzzaman 2015). Auta & Hameed (2012) reported methylene blue removal of 160, 298 and 377 mg/g at concentrations of 50, 100 and 200 mg/L, respectively, using waste tea activated carbon/chitosan composite beads in a column with 3.6 cm bed thickness and 5 mL/min flow rate. Makrigianni et al. (2017) reported the performance of acid-treated, pyrolytic tyre char as 2.08, 3.79 and 3.85 mg/g at concentrations of 10, 20 and 40 mg/L, respectively, in a 15 cm tall column operating at 100 mL/min. Similarly, Auta & Hameed (2014) showed methylene blue adsorption onto chitosan/clay composite of 142 mg/g, at a concentration of 200 mg/L in a 3.6 cm bed at 5 mL/min flow rate.

Modified chitosan has gained wide attention as an effective adsorbent due to its versatility in water, especially when blended with a robust material to form a composite (Li & Tang 2016). Modified chitosan contains high concentrations of amino and hydroxyl functional groups that have significant affinity for positively charged water pollutants (Zdarta et al. 2015; Tondwal & Singh 2018). As far as is known, however, no work has been done to date on methylene blue adsorption by crab shell chitosan/neem leaf composite (chitosan composite) in column-mode. Crab shell and neem leaves are available at little cost as primary sources for chitosan composite and offer a new alternative to current adsorbents. Therefore, this study was aimed at assessing the adsorption dynamics of chitosan composite on methylene blue removal. The dynamics of the column-mode process were analysed and described using the Adams-Bohart, Thomas and Yoon-Nelson models.

Crab shell and neem leaves were obtained from Auchi, Nigeria. All chemicals used were of analytical grade: sodium hydroxide pellets (99%, Merck, Germany), hydrochloric acid (37%, Fisher Scientific, USA), oxalic acid pellets (99%, Merck, Germany) and methylene blue powder (98.5% Merck, Germany).

Crab shell was deproteinised, demineralised and deacetylised to produce chitosan, from which the 300 μm material was screened (Asokogene et al. 2019). Neem leaves were rinsed with distilled water, oven-dried at 110 °C for 150 minutes, ground to a fine powder and heat-treated in a muffle furnace at 250 °C for 180 minutes. Thirty grams of chitosan was mixed with 10% (w/v) oxalic acid solution, and stirred for 1 hour at 50 °C and 200 rpm, before 50 g of powdered neem leaf was added slowly, stirred continuously for 2 hours and allowed to stand for 24 hours. The chitosan composite was removed from the solution and washed with distilled water. Next, it was soaked in 0.5% (w/v) sodium hydroxide solution for 3 hours. Finally, the material was rinsed with distilled water and dried in an oven at 110 °C.

The chitosan composite was characterized for textural properties using a Micromeritics ASAP 2010 analyzer, and surface chemistry using a Nicolet ISI 10 Fourier transform infrared (FTIR) spectrometer (both Thermo Scientific, USA).

The chitosan composite has a specific surface of 258 m²/g, with 65.4% mesoporosity and mean pore diameter of 2.07 nm. The FTIR spectrum of chitosan composite shows an absorption band at 3,679–3,332 cm⁻¹, corresponding to O – H hydrogen bond and N – H stretching vibrations of primary and secondary amine/amide groups in the material (Asokogene et al. 2019).

Figures 1–3 show the effect of operating conditions on column-mode methylene blue adsorption onto chitosan composite as presented in breakthrough curves. The column parameters are summarized in Table 1.

The effect of adsorbate concentration on dye adsorption under fixed conditions – flow rate 2.17 mL/min, bed thickness 5.0 cm and pH 4.8 ± 0.3 – is presented in Figure 1. The breakthrough curve remains close to the abscissa for 155 minutes for influent concentration 50 mg/L. At 200 mg/L, a shorter breakthrough time of 64 minutes was observed. Accordingly, a throughput of 3,012 mL can be treated at the lower dye concentration. The values of q_total and M_total increased from 55.6 to 63.3 mg, and 122 to 238 mg, respectively, with the increase in dye concentration, while the removal efficiency decreased from 45.6 to 26.6% due to the higher residual concentration in the column effluent. A similar dynamic pattern was reported by López-Cervantes et al. (2018). The operation embraced the difference in solute mass transfer because of the concentration gradient between 50 and 200 mg/L (Karimi et al. 2012; López-Cervantes et al. 2018). The adsorbent was exhausted after 1,108 and 195 min for the 50 and 200 mg/L influents, respectively.

The effect of bed thickness on methylene blue adsorption dynamics at C_o = 200 mg/L is shown in Figure 2. Increasing the bed thickness from 2.5 to 5.0 cm increased the adsorption capacity from 38.4 to 63.3 mg/g. The greater thickness provided numerous active sites and more residence time for adsorbent-adsorbate interaction, thereby increasing adsorption capacity (López-Cervantes et al. 2018).

Figure 3 shows the effect of flow rate on methylene blue adsorption dynamics. Adsorption was rapid at both flow rates, possibly due to the abundance of vacant active sites and ionic interactions. As adsorption continued, there was a reduction in the adsorption rate as more sites were occupied (Afroze et al. 2016). An adsorption capacity of 76.8 mg/g was recorded at 2.90 mL/min flow rate, suggesting the prevalence of mass transfer and protonation of amine groups at higher flow rates in promoting dye molecule diffusion onto chitosan composite (Auta & Hameed 2012).

• K_Th (mL/min. mg) is the Thomas rate constant, q_T (mg/g) the equilibrium capacity, m (g) the mass of adsorbent, and t (min) the time;

• K_YN (min⁻¹) is the Yoon-Nelson rate constant, (min) the time required for 50% adsorbate breakthrough; and,

• k_AB (L/mg.min) is the Adams-Bohart rate constant, u (cm/min) the linear velocity, Z (cm) the bed thickness, N the maximum adsorption capacity, C_o the influent concentration and C_t the adsorption concentration at time, t.

The column-mode parameters from these models were solved by non-linear regression using MS Excel, and the values are summarized in Table 2.

Generally, the adsorption dynamics data fitted well into all models as reflected in the high correlation of determination (R²) values, hence their applicability in describing the breakthrough data. The Thomas model analysis shows a decreasing rate constant (K_TH) from 0.204 to 0.128 mL/min.mg, with increasing flow rate from 2.17 to 2.90 mL/min, while the equilibrium capacity, q_T, increased from 47.7 to 79.2 mg/g. Similarly, increasing the bed thickness from 2.5 to 5.0 cm caused K_TH to decrease from 0.298 to 0.204 mL/min.mg and q_T to increase from 37.7 to 47.7 mg/g. These results signify that the adsorbate residence time in the column and solute mass transfer onto the active sites was sufficient. Therefore, methylene blue adsorption onto chitosan composite increased at higher flow rate, greater bed thickness and lower influent concentration. Similar results were reported by others (Altufaily et al. 2019; Talib et al. 2018). Generally, the breakthrough simulation for all operating conditions studied obeyed the Thomas model, as indicated by the close agreement of q_T with experimental q_o.

The Yoon-Nelson model shows that increasing the flow rate and bed thickness reduces the rate constant, K_YN, and increases the time required for 50% breakthrough. Increasing the influent concentration, however, increases K_YN and reduces τ, suggesting increased competition for adsorption sites and higher adsorption capacity (Afroze et al. 2016; Altufaily et al. 2019).

The Adams-Bohart model shows that the rate constant, K_AB, decreases and the saturation concentration, N_o, increases with increasing flow rate and bed thickness. However, increasing influent concentration increases both K_AB and N_o, suggesting that the kinetics are concentration driven. Talib et al. (2018) reported similar results.

Table 3 is a comparison of methylene blue adsorption by chitosan composite with other adsorbents in column-mode operation from selected literature sources at varying influent concentrations and flow rates. Generally, the chitosan composite displays a competitive performance with the adsorbents listed in Table 3. The ready availability of the natural resources for the chitosan composite and its excellent adsorption potential render it a promising low-cost alternative for dye wastewater treatment.

A chitosan composite was used to study methylene blue adsorption dynamics. The mesoporous adsorbent has a specific surface of 258 m²/g and its performance with respect to methylene blue was comparable to that of other adsorbents reported in literature. Adsorption capacity was affected noticeably by column operating conditions, specifically influent concentration, bed thickness and flow rate. The maximum adsorption capacity recorded was almost 77 mg/g at C_o = 200 mg/L. The breakthrough behaviour reveals the importance of external and internal diffusion, and a simultaneous steady-state process of intraparticle diffusion and possibly ionic interaction as the rate controlling mechanisms.

This work was supported by Tertiary Education Trust Fund (TETFund) of Nigeria through an Academic Staff Training and Development (AST&D) grant, and the Ministry of Education of Malaysia and Universiti Teknologi Malaysia through Fundamental Research Grant Scheme (FRGS) No. 4F995.

The authors of this work have no conflict of interest to declare.

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