Effluents containing inorganic contaminants are releasing into the environment untreated despite being hazardous to man and environment. It is costly and unsustainable to use conventional methods to remove them from dilute aqueous solution. Adsorption involving granular activated carbon is an alternative method for treating such effluents. Granular activated carbon is structurally strong, highly resistance to attrition and wearing, large and can easily separate from the effluents. However, its surface is highly hydrophobic and has little surface charge thereby reducing its adsorption capacity for anion or cation. This article reviews surfactant modification of activated carbon to enhance its adsorption capacity for inorganic contaminants and key factors affecting the adsorption efficiency. They include initial concentration of contaminants, contact time, solution pH, solution temperature, adsorbent concentration, ionic strength, competing ions, type of surfactant, and surfactant concentration. The modified activated carbon usually shows maximum contaminant uptake around its critical micelles concentration. Surfactant modification reduces specific surface area and/or micro pore volume but hot NaOH or HNO 3 treatment before surfactant modification minimises this drawbacks and increases the net surface charge. Overall, surfactant modification is a simple but efficient method of enhancing adsorption capacity of activated carbon for removing anion or cation from aqueous solution. However, a handful publication is available on the regeneration of the spent (saturated) surfactant modified activated carbons. Hence, more research efforts should be directed towards proper regenerating reagents and the optimise conditions such as contact time, concentration, and temperature for regenerating spent modified activated carbons.
Massive industrial, domestic, agricultural, medical and technological applications of heavy metals contribute to their prevalence in the environment [
According to United State Environmental Protection Agency, National Primary Drinking Water Regulations [
Despite these potential health hazards of inorganic contaminants to man and negative impact to the environment, over 80% of the wastewater globally generated and over 95% in some least developed countries is released into the environment untreated [
The conventional methods of removing water contaminants from effluents include ion exchange, evaporation, chemical precipitation, membrane separation, chemical oxidation or reduction, electrochemical treatment, reverse osmosis [
Similarly, adsorption involving nano-adsorbents or powdered activated carbon is hardly used due to operational problems. This includes complex and costly synthesis, turbidity inducement and separation difficulty [
Considering the fact that activated carbon’s surface is highly hydrophobic [
This article reviews the use of surfactant modified activated carbons for adsorption of anions or cations from aqueous solutions, environmental factors and surfactant properties required to improve the process efficiency, drawbacks of the surfactant modified activated carbon and ways to minimise them. The author also suggests areas required more research effort.
Granular activated carbon is structurally strong, highly resistance to attrition and wearing [
Previous investigations show that surfactant surface modification of activated carbon is possible and effective for enhancing its adsorption capacity for removing inorganic contaminant from aqueous media.
Anh et al. [
Contaminant | Surfactant, concentration. (g/L) | Solution pH | Initial conc. or range (mg/L) | Contact time (min) | Agitation (rpm) | Temp. (˚C) | Kinetic model | Isotherm | Contaminant removed (% or mg/g) | References |
---|---|---|---|---|---|---|---|---|---|---|
Cd (II) | SDS, 10.1, 4.28*CMC | 6 | 100 | 120 | 100 | 20 | Pseudo second order | Freundlich | 22.3 | Ahn et al. [ |
SDBS, 10.1 10.64*CMC | 6 | 100 | 120 | 100 | 20 | Pseudo second order | Freundlich | 17.53 | Ahn et al. [ | |
DSS, 10.1 5.94*CMC | 6 | 100 | 120 | 100 | 20 | Pseudo second order | Freundlich | 17.31 | Ahn et al. [ | |
Virgin activated carbon | 6 | 100 | 120 | 100 | 20 | Pseudo second order | Freundlich | 2.47 | Ahn et al. [ | |
Cd (II) | SDBS, 30.7 73.4*CMC | 4.5 - 7.2 | 10 - 100 | 1440 | - | 25 | ? | Langmuir | 44.2a | Sun et al. [ |
Virgin activated carbon | 4.5 - 7.2 | 10 - 100 | 1440 | - | 25 | ? | Langmuir | 6.8 | Sun et al. [ | |
Co (II) | SDS, 3.54 1.5*CMC | 7 | 20 - 100 | 90 | 250 | 40 | Pseudo second order | Langmuir | 51 | Kakavandi [ |
Bromate | CTAC, 0.64 1.26*CMC | 3 | 2 - 20 | 1440 | - | 25 | Pseudo second order | Langmuir | 38.02 | Chen et al. [ |
Virgin activated carbon | 3 | 2 - 20 | 1440 | - | 25 | Pseudo second order | Langmuir | 7.02 | Chen et al. [ | |
Bromate | CTAC, 0.64 2*CMC | - | 1 - 100 | 720 | - | - | Pseudo second order | Langmuir | 35.8 | Farooqet al. [ |
Bromate | CPC, 0.68 2*CMC | - | 1 - 100 | 720 | - | - | Pseudo second order | Langmuir | 34.2 | Farooqet al. [ |
CTAB, 0.729 2*CMC | - | 1 - 100 | 720 | - | - | Pseudo second order | Langmuir | 13.1 | Farooq et al. [ | |
Virgin activated carbon | - | 1 - 100 | 720 | - | - | Pseudo second order | Langmuir | ~13.1 | Farooq et al. [ | |
Nitrate | CTAB, 10.9 33.3*CMC | 5.6 | 100 | 5 | 250 | 25 | Pseudo second order | Langmuir | 83.3; 60% | Allalouet al. [ |
Virgin activated carbon | 5.6 | 100 | 5 | 250 | 25 | Pseudo second order | Langmuir | 14% | Allalou et al. [ | |
Nitrate | CTAB, 0.164 0.5*CMC | 7 | 40 - 200 | 120 | 200 | 25 | Pseudo second order | Langmuir | 21.5b | Mazarji et al. [ |
Nitrate | CPMG, 2.5 | 6.5 - 6.8 | 25 - 376 | 90 | 150 | 23 | Pseudo second order | Langmuir | 26 | Cho et al. [ |
Virgin activated carbon | 6.5 - 6.8 | 25 - 376 | 90 | 150 | 23 | Pseudo second order | Langmuir | 14.3 | Cho et al. [ | |
Cr (VI) | CPMG, 2.5 | 6.5 - 6.8 | 25 - 376 | 90 | 150 | 23 | Pseudo second order | Langmuir | 81 | Cho et al. [ |
Virgin activated carbon | 6.5 - 6.8 | 25 - 376 | 90 | 150 | 23 | Pseudo second order | Langmuir | 55 | Cho et al. [ | |
Cr (VI) | HDTMA, 0.168 0.5*CMC | - | 10 - 200 | 180 | 150 | 20 | Pseudo second order | Langmuir | 4.06 | Choi et al. [ |
CPC, 0.161 0.5*CMC | - | 10 - 200 | 180 | 150 | 20 | Pseudo second order | Langmuir | 3.70 | Choi et al. [ | |
Virgin activated carbon | - | 10 - 200 | 180 | 150 | 20 | Pseudo second order | Langmuir | 1.28 | Choi et al. [ | |
Perchlorate | CTAC, 1.6 5*CMC | ~6.7 | 5 - 50 | 120 | 120 | 60 | Langmuir | 55.3 | Tang et al. [ |
aTreatment with HNO3 before SDBS modification; bTreatment with NaOH before CTAB modification.
22.3 mg/g > sodium dodecyl benzene sulfonate (SDBS) modified activated carbon, 17.53 mg/g > dioctyl sulfocuccinate sodium (DSS) modified activated carbon, 17.31 mg/g > virgin activated carbon, 2.47 mg/g. Similarly, Kakavandi et al. [
Sun et al. [
Surfactant modified activated carbons equally work effectively for enhancing adsorption of anions from aqueous solutions. Chen et al. [
Other researchers reported enhancement in percentage nitrate removed by CTAB modified activated carbon [
Cho et al. [
Surfactant | Virgin activated carbon | Pre-treated activated carbon | Modified activated carbon | References | |||
---|---|---|---|---|---|---|---|
BET specific surface area (m2/g) | Micropore volume (cm3/g) | BET specific surface area (m2/g) | Micro pore volume (cm3/g) | BET specific surface area (m2/g) | Micropore volume (cm3/g) | ||
CTAB | 888 | 0.376 | 901a | 0.400b | 722c | 0.310d | Mazarji et al. [ |
SDBS | 158.1 | 0.0804 | 185.07e | 0.0937f | 131.4g | 0.0729h | Sun et al. [ |
CTABr | 1407 | - | - | - | 608.9 | - | Allalou et al. [ |
CPC | 822.7 | - | - | - | 608.9 | - | Farooq et al. [ |
CTAB | 822.7 | - | - | - | 600.1 | - | Farooq et al. [ |
CTAC | 822.7 | - | - | - | 571.3 | - | Farooq et al. [ |
aBET specific surface area after NaOH treatment, bMicropore volume after NaOH treatment, cBET specific surface area after NaOH treatment plus CTAB modification, eBET specific surface area after HNO3 treatment, fMicropore volume after HNO3 treatment, gBET specific surface area after HNO3 treatment plus SDBS modification, hMicropore volume after HNO3 treatment plus SDBS modification.
BET specific surface of 1407 m2/g reduced to 569 m2/g after CTAB modification. Similarly, Farooq et al. [
However, treatment with NaOH or HNO3 prior to surfactant modification has been shown to minimise the reduction in the BET specific surface surface or micro pore volume and ultimately increase the net surface charge of the modified activated carbons. Mazarji et al. [
Similarly, Sun et al. [
mg/g > virgin activated carbon, 6.78 mg/g. The modified and activated carbons’ performances seems to follow the same order as the net surface charge with HNO3-SDBS modified activated carbon, −39.7 > HNO3 modified activated carbon, −28.1 > SDBS modified activated carbon, −22.1 > virgin activated carbon, −18.6. Although there was an interchange in place between HNO3 modified activated carbon and SDBS modified activated carbon, nevertheless combination of HNO3 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) [
2 ROH + M 2 + ↔ ( RO ) 2 M + 2 H + (1)
2 RCOOH + M 2 + ↔ ( RCOO ) 2 M + 2 H + (2)
2 RSO 3 Na + M 2 + ↔ 2 RSO 3 M + 2 Na + (3)
RSO 3 Na + H 2 + ↔ RSO 3 H + Na + (4)
Equation (1) and Equation (2) indicate that cations can bind with little active binding groups present originally on the virgin activated carbon if they are not covered by the surfactant after modification. Equation (3) indicates that cations can bind with negatively charged group of the anionic surfactant anchored onto the surface of the activated carbon (ion exchange). Equation (4) indicates possible competitive or inhibition binding between cations and hydrogen ions with negatively charged group of the anionic surfactant.
Possibility of these reactions is further supported by the pH profile observed by Ahn et al. [
Sun et al. [
Possible adsorption mechanism for the uptake of anions by cationic modified activated carbons can also be made from empirical findings. Farooq et al. [
Sometimes amount of counter anions exchanged varies with the initial concentration of the targeted anions. Tang et al. [
Overall, mechanisms of adsorption of cation (anion) onto activated carbon modified by anionic (cationic) surfactant include ion exchange, electrostatic attraction, physical adsorption or surface complexation depending on the quantity, activity and masking of the surface functional groups originally present on the virgin activated carbon before surfactant modification.
Factors affecting percentage ion removed and/or specific ion uptake by surfactant modified activated carbon can be broadly categorised into environ mental factors and surfactant properties.
The environmental factors include process parameters such as initial concentation of contaminants, contact time, solution pH, solution temperature, adsorbent concentration, ionic strength and competing ions.
The pH of the sorbate solution is one of the key parameters affecting adsorption. It influences the net surface charge of the binding sites, solution chemistry of the target metal in terms of hydrolysis or complexation [
The value of the zeta potential is used to characterise the net surface charge on an adsorbent. Positive zeta potential indicates positive net surface charge while negative zeta potential means negative surface charge. At every pH the zeta potential of cationic surfactant modified activated is always higher in positive value than the corresponding virgin activated carbons [
Xu et al. [
Tang et al. [
Increase in the net negative surface charge of both virgin and cationic surfactantmodified activated carbon as pH increases causes repulsion between the negatively charged surface and the anions; hence reducing its uptake at such high pH. In addition, the residual hydroxide ions, OH – produced due to higher pH compete with the targeted anions for binding on the adsorbent surface, thereby reducing the amount of anions removed. As pH decreases below pHpzc, the net positive surface charge increases allowing more of the anions to be attracted onto the adsorbent surface. This usually increases the specific anions uptake by cationic modified activated carbon. However, excessive increase in the pH far below pHpzc reduced specific uptake of nitrate by CTAB modified activated carbon [
Anionic surfactant modification enhances the net negative surface charge of virgin activated carbons [
This behaviour was attributed partly to reduction in competition between H+ and Cd (II) for binding on the same sites as pH increases [
Percentage inorganic contaminants removed by adsorbents usually decreases as initial ion concentration increases [
In contrast, adsorption capacity usually increases with increase in initial concentration [
For a fixed mass of adsorbent, at low initial adsorbate concentration, active binding sites are under saturated but tend towards saturation as the initial concentration increases [
Surfactant modified activated carbon concentration is another important factor affecting the percentage contaminant removed and the specific contaminant uptake. At constant adsorbate concentration, increase in concentration of the adsorbent increases the percentage contaminant removed but decreases the specific contaminant uptake.
Kakavandi et al. [
Rising in percentage adsorbate could be attributed to more active binding sites and/or enhance surface area available as the adsorbent concentration increases for a fixed initial contaminant concentration [
Percentage contaminant removed and specific contaminant uptake by surfactant modified activated carbon usually increased with contact time until equilibrium. At optimum surfactant modification conditions, the adsorption capacity of the modified activated is usually greater than that of virgin activated carbon. However, adsorption rate of such modified activated carbons has no definite pattern. Surfactant modification could increase adsorption rate [
Specifically, Farooq et al. [
In contrast, Chen et al. [
Equilibrum times reported by different authors also differ. Allalou et al. [
Of note is that some authors extend the contact time during isotherm studies above the equilibrium time obtained during kinetics studies. This explains a wide range (5 - 1440 min) in the contact time listed in
Many authors conducted adsorption of inorganic contaminant onto surfactant modified activated carbon at ambient temperature [
Tang et al. [
Cho et al. [
Increase in ionic strength (NaCl concentration) suppresses ion exchange but enhances non-electrostatic (hydrophobic) attraction [
The rapid reduction could be explained by suppression of ion exchange between perchlorate, ClO 4 − and CTAC on the surface of CTAC modified activated carbon suggesting that the main sorption mechanism by this system is ion exchange while the slight reduction at 50 mmol/L NaCl could be explained by occurrence of non-electrostatic interaction accounting for the perchlorate adsorption at this region [
There is also an interaction between NaCl concentration and the contact timeon suppression of ion uptake by the surfactant modified activated carbon. Xu et al. [
Industrial effluents hardly contain single inorganic ion. The co-ions will compete with the targeted ions for binding sites if they have preference for the same sites [
Mazarji et al. [
Type of surfactant and surfactant concentration also contribute to the overall effectiveness of the modified activated carbons for adsorption of cations or anions from aqueous solution.
Surfactant can be broadly classified into anionic, cationic and non-ionic. The targeted ion determines the type of surfactant required for modifying the activated carbon. This is because investigation into the mechanism of adsorption of ions by a surfactant modified activated carbon suggests that ion exchange is usually the main mechanism [
The surfactant concentration used for modification of activated carbon also contributes to its adsorption capacity. Chen et al. [
Similarly, Kakavandi et al. [
Adsorption capacity of cationic or anionic surfactant modified activated carbon increase as surfactant concentration increases due to enhancement in their net surface positive or negative charge respectively. However, net surface charge of cationic surfactant modified activated carbons did not increase significantly beyond their CMC [
In addition, reduction of the micro-pore volume above CMC has high tendency of masking the active binding sites originally present on the virgin activated carbon thereby reducing their contribution to physical adsorption, surface complexation and electrostatic interaction to the main ion exchange. Few researchers fixed surfactant concentration below CMC [
Alkali (NaOH) and acid (HNO3) are the common reagents for treating activated carbon before surfactant modification to enhance their adsorption capacity.
Mazarji et al. [
Sun et al. [
Stability or leaching of surfactant modified activated carbon in aqueous solution should be considered during selection. This is important to ensure effluent quality and minimise secondary contaminants via surfactant desorption. Mazarji et al. [
In contrast, Parette et al. [
Regeneration of spent (saturated) modified activated carbons will minimise disposal. It will also make the process more environmental friendly, preserve our resources and more cost effective. Findings show that surfactant modified activated carbon can be regenerated. Cho et al. [
The specific nitrate uptake by the CPMG for the three consecutive adsorption/desorption cycles reduced from 21.4mg/g (for the fresh CPMG-activated carbon) to 19.4, 15.9, and 14.9 mg/g respectively. The regeneration efficiencies were 90.7%, 82%, and 93.7% respectively. The specific Cr (VI) uptake by the CPMG reduced from 59 mg/g (for the fresh CPMG-activated carbon) to 51.8, 49.3 and 45.5 mg/g respectively. The regeneration efficiencies were 87.8%, 95.2% and 92.3% respectively. At the 0.05 M NaOH (equivalent to pH 12.7) the net surface charge on the CPMG-activated carbon will be negative which will enhance desorption of the adsorbed negatively charges nitrate and Cr (VI) ions.
Tang et al. [
Surfactant concentration used for modification of activated carbon contributes to its specific contaminant uptake. The modified activated carbon usually shows maximum contaminant uptake around its critical micelle concentration.
Well-developed micro pores are required to allow anchoring of enough surfactant for high net surface charge and to minimise reduction in the specific surface area/micro pores volume after surfactant modification. Hot NaOH or HNO3 treatment prior to surfactant modification has been shown to minimise surface area reduction and/or increase net surface charge of the modified activated carbons.
Solution pH is one of the key environmental parameters affecting adsorption capacity of the modified activated carbons and its variation can be used for regenerating the spent (saturated) modified activated carbon. Adsorption capacity of cationic modified activated carbon for targeted anion increases with decrease in pH below pHpzc but decreases with increase in pH above pHpz. Adsorption capacity of anionic modified activated carbon for targeted cation increases with increase in pH above pHpzc but decreases with decrease in pH below pHpzc.
Overall, surfactant modification is a simple but efficient method of enhancing adsorption capacity of activated carbons for removing anions or cations from aqueous solution. However, a handful publication is available on the regeneration of the spent (saturated) surfactant modified activated carbons. Hence, more research efforts should be directed towards proper reagents and the optimised conditions such as contact time, concentration, and temperature for regenerating such modified activated carbons.
The author declares no conflicts of interest regarding the publication of this paper.
Salam, K.A. (2019) Assessment of Surfactant Modified Activated Carbon for Improving Water Quality. Journal of Encapsulation and Adsorption Sciences, 9, 13-34. https://doi.org/10.4236/jeas.2019.91002