Nitric acid-anionic surfactant modified activated carbon to enhance cadmium(II) removal from wastewater: preparation conditions and physicochemical properties

Abstract

The authors used a nitric acid (HNO3)-sodium dodecyl benzene sulfonate (SDBS) method to modify a lignite-based activated carbon. These modified carbons were appraised for their removal of Cd(II) from aqueous solutions. Response surface methodology was employed to optimize the preparation factors including nitric acid concentration CN, temperature T and SDBS concentration CS. Statistical analysis indicated that the interaction of CN and CS incurred the most effect on the maximum cadmium adsorption capacity (Qm). The optimal Qm appeared at CN = 3.29 mol/L, T = 76 °C and CS=30,700 mg/L. The optimal protocol achieved 44.21 mg/g Qm for Cd(II) which was about 7 times larger than for this pristine lignite activated carbon (LAC) (6.78 mg/g). The physical-chemical properties of the modified activated carbons following each synthesis step were characterized relative to their surface area, oxygen functionality, and external surface charge. It was confirmed that the developed surface area, functional groups and negative charges were mainly responsible for the higher adsorption capacity for the LAC that have been more favorably tailored by this HNO3-SDBS protocol.

Assessment of Surfactant Modified Activated Carbon for Improving Water Quality

“…Similarly, Sun et al [24] observed that the BET specific surface area and micro pore volume of a virgin activated carbon, 158 m 2 /g and 0.0804 cm 3 /g increased to 185.07 m 2 /g and 0.0937 cm 3 /g respectively after HNO 3 treatment but reduced to 131.4 m 2 /g and 0.0729 cm 3 treatment with SDBS modification resulted into an increase in the net surface charge of the virgin activated carbon and enhancement in the adsorption capacity.…”

“…Surface characterisation of virgin activated carbons shows that they have functional groups including carboxyl (R-COOH), phenolic (R-OH) and carbonyl (R = O) [23] [24]. The carbonyl group can complex with inorganic contaminant while the deprotonated acidic functional group can bind heavy metals ions.…”

“…Anionic surfactant modification enhances the net negative surface charge of virgin activated carbons [23] [24] and its surface become more negatively charged as the pH increases. Hence, adsorption of cations by anionic surfactant modified activated carbons increases with pH due to the attraction between the negatively charged surface and the targeted cations.…”

“…Increase in micro pores could increase nitrate uptake via hydrophobic interaction such as physical adsorption, surface complexation and non-electrostatic interaction [61]. NaOH pre-treatment improves removal/solubility of most organic components attached to virgin activated carbon into water by weakening their attractive van der Waals forces [24].…”

“…Considering the fact that activated carbon's surface is highly hydrophobic [17] [18] surfactant can be anchored on it using its hydrophobic group while its hydrophilic group tends towards the aqueous phase and ultimately determines its net surface charge [17] [19]. Modification of activated carbon with cationic surfactant makes its net surface charge positive [20] [21] [22] but using anionic surfactant makes it negative [23] [24]. In addition, keeping the pH of surfactant modified activated carbon system below or above pHpzc (pH at which its surface charge equals zero) can alter the net surface charge from positive to negative or vice versa [20] [21] [22].…”