Mercury sorption by silica/carbon nanotubes and silica/activated carbon: a comparison study

Mercury removal from wastewater has received significant attention due to its toxicity and thus high impact on the environment and public health. Adsorption technology, using ion-exchange resins, activated charcoal, nanomaterials and ion chelating agents immobilized on inorganic supports as adsorbents, is one of the most popular methods (Teng et al. 2011; Vasudevan et al. 2012; Wang et al. 2014; Mehdinia et al. 2015). Mercury is widely distributed in the environment from various sources including industries such as pigments, metallurgy, gilding copper, and instrumentation, oil, petrochemicals, recovery of gold, manufacture of chlorine and sodium hydroxide by electrolysis of brine and in many industries (Silva et al. 2010). The crude oil and liquid condensate can contain some quantities of mercury, ranging from nil to over 10 ppm. The mercury shortens the life of hydrogenation catalysts by deactivation and part of the mercury can also end up in the wastewater, causing contamination of the refinery site and, particularly, the wastewater treating facilities including the settling pond (Nasirimoghaddam et al. 2015).

Nanomaterials have wide applications as sorbents for water purification as they have high efficiency and capacity for heavy metal ions in aqueous solutions. Carbon-based nanomaterials have generated great interest in their use as adsorbents for the removal of pollutants from water/wastewater as they are stable, have limited reactivity, wide surface area, and are strong antioxidants (Halem et al. 2007; Peters et al. 2008; Sorlini & Gialdini 2014; Chawla et al. 2015; Naghizadeh 2015). For example, hybrid ligands-modified activated carbon, amine-modified activated carbon, multi-walled carbon nanotubes and chitosan/carbon nanotube composite beads (Zhu et al. 2009; Shadbad et al. 2011; Shawky et al. 2012) have also been reported for their efficiency as adsorbents for mercury removal.

Because of their stability and large specific surface area, carbon nanotube (CNT) and activated carbon (AC) have attracted much attention (Saleh 2011; Gupta & Saleh 2013). On the other hand, silica has chemical stability and versatility in surface modification. Silica with different functionalities was studied as sorbents for pollutants sorption from waters (Chen et al. 2008; Yin et al. 2010).

Here, we report on the synthesis of silica/CNT and silica/AC and their sorption performance for mercury removal from aqueous solutions. The effects of contact time, initial concentration, pH and equilibrium isotherms on the process were investigated.

Carbon nanotubes (CNT) and activated carbon (AC) were prepared and activated as per the work reported earlier (Saleh 2011; Saleh & Al-Saadi 2015). The mercury standard stock solution (1,000 mg/L) Hg²⁺ was used. The solution was diluted to different initial concentrations as required. The initial pH of the tested solutions was adjusted to the desired value by using nitric acid or sodium hydroxide.

The prepared composites were characterized for the morphology using scanning electron microscopy (SEM) and element mapping, and structural properties using energy dispersive X-ray spectroscopy (EDX) and Fourier transform infrared spectroscopy (FTIR). A thermogravimetric analyzer (TGA) was used to evaluate the thermal stability of the prepared composites. Thermo Electron Corporation NXR FT-Raman module Nicolet 6700 FT-IR spectrometer in a region of 4,000–400 cm^–1 was used to characterize the structure of the prepared silica/CNT and silica/AC. A thermal analyzer (STA 429) (Netzsch – Germany) at a constant heating rate of 10 °C/min under nitrogen flow was used to determine the thermal stability of the composites.

In order to specify the electrical neutrality of the adsorbent at a particular value of pH under aqueous solution conditions, point of net zero charge (pHzpc) was determined using the pH titration method. Briefly, 11 solutions with pH values of 1.0 to 12.0 were prepared. Then, 0.3 g of silica/CNT or silica/AC was added into each bottle and the final pH was measured after 48 h. The pH_pzc is defined as the point where the curve pH_final vs. pH_initial crosses the line pH_final = pH_initial (Órfão et al. 2006).

A mercury analyser was used to monitor the concentration of Hg²⁺ in the aliquots. Concentration of metal ions in real wastewater samples were monitored by inductively coupled plasma mass spectrometry (ICP-MS) model ICP-MS XSERIES-II Thermo Scientific with the following instrumental parameters; RF power 1,404 W, plasma gas flow 13 L/min, nebulizer gas flow 0.95 L/min, auxiliary gas flow 0.7 L/min, quartz pneumatic nebulizer, spray chamber: glass with peltier cooling, three replicates, acquisition mode; pulse counting, dwell time 10 ms, sweeps/reading 100 and acquisition parameters of scanning mode peak hopping, dwell time 300 ms and integration mode: peak area.

Specific amounts of silica/CNT or silica/AC were added into 20 mL of Hg²⁺ solution in plastic containers which were placed in a bath shaker at 150 rpm. The effect of the temperature was investigated by adjusting the temperature of the bath. Experimental parameters such as the effects of pH, initial Hg²⁺ concentration, contact time and temperature were studied. A comparison was performed to evaluate the performance of the silica/CNT comparing with silica/AC. This study was conducted by adding an equivalent amount of each adsorbent into 20 mL of Hg²⁺ solution.

The column was packed with the silica/CNT or silica/AC. Then the prepared Hg²⁺ solutions were passed through them in the bed to study their adsorption capacity. The column diameter and length used in every experiment was fixed constant. The different layer thickness of the adsorbent and the flow rate of the solutions were used as specified.

The Hg-loaded silica/CNT or silica/AC were eluted by stirring with 1M HNO₃ to desorb Hg²⁺. The silica/CNT or silica/AC was washed with de-ionized water and allowed to dry and then reused in adsorption processes.

C_o = initial mercury concentration in mg/L; C_t or C_e = concentrations at time or equilibrium in liquid phase.

The pseudo-first-order, second-order kinetic models were used to express the mechanism of solute sorption onto a sorbent. In order to design a fast and effective model, investigations were made on adsorption rate. For the examination of the controlling mechanisms of adsorption process, such as chemical reaction, diffusion control and mass transfer, the intraparticle diffusion model was used to test the experimental data.

The obtained results of models parameters are listed in Table 2. Considering the correlation coefficient (R²), it can be observed that the experimental data fit better to the Freundlich isotherm, Figure 8(b), compared with the other models which means that the adsorption process are of heterogeneous surface with interactions between the adsorbed molecules. The value of k_f in the case of using silica/CNT is higher than silica/AC meaning that the silica/CNT is of better adsorption capacity than silica/AC. The n value gives an indication for the favourability of the adsorption process. The values of n > 1 represent a favourable adsorption condition. However, the value of n in the case of using silica/CNT is higher than silica/AC meaning that the silica/CNT is of more favourable adsorption condition than silica/AC.

The free energy change was calculated at 296, 316 and 336 K and was found to be –20, 21, 23 kJ/mol when silica/CNT was used and –13, 14, 16 kJ/mol when silica/AC was used. The decrease in its values with increasing temperature indicates the favorable adsorption at higher temperature. The positive standard enthalpy change ΔH⁰ of 13 and 7 kJ/mol suggests the adsorption of Hg²⁺ is endothermic and the positive standard entropy change reflects the affinity of the adsorbents towards Hg²⁺.

The reusability of the silica/CNT and silica/AC is important in real applications from both an economic and environmental point of view. Therefore, reusability must produce a small volume of metal concentrate suitable for mercury recovery without damaging the adsorbent nature. Here, several cycles of alternating adsorption and desorption stages with silica/CNT or silica/AC were performed. Under the same selected operation conditions, the readsorption of Hg²⁺ reached percentages around 99% when silica/CNT was used, while it reached around 92% when silica/AC was used. This shows that the silica/CNT is highly effective for the adsorption of Hg²⁺ ions from aqueous solutions.

We reported the fabrication of silica/CNT and silica/AC. The prepared materials were characterized by SEM, element mapping, EDX and FTIR. The elemental mapping indicated the uniform distribution of the silica within the composite. The kinetic data for the adsorption process obeyed a pseudo-second-order kinetic model and Freundlich isotherm model, with silica/CNT of higher capacity than silica/AC. The finding confirms that intraparticle diffusion is involved in the adsorption process. The effectiveness of the silica/CNT was compared with silica/AC using a packed column for the removal of Hg²⁺. The silica/CNT showed better performance with a longer breakthrough point than silica/AC. Therefore, the materials are promising candidates for heavy metal ion, especially mercury, removal. Further research on these adsorbents is required toward other pollutants.

Support from the Chemistry Department and King Fahd University of Petroleum and Minerals is gratefully acknowledged. The author would like to acknowledge the support provided by the Deanship of Scientific Research (DSR) at King Fahd University of Petroleum & Minerals (KFUPM) for funding this work through project No. JF121009.