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 [30] . pH of the solution also contributes to metal precipitation, speciation and its availability for adsorption [31] . Isoelectric point pH ( pH IEP ) or point of zero charge ( pH PZC ) is a useful guide to find an ideal pH for maximum inorganic contaminant uptake. This is the pH of the solution at which the net surface charge on the adsorbent equals zero. Solution pH above pH PZC makes the net surface charge on the adsorbent negative while solution pH below pH PZC makes the net surface charge positive [32] . Electrokinetic studies to estimate the isoelectric point pH ( pH IEP ) is documented in the literature [33] [34] .

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 [20] [21] [22] while that of anionic surfactant usually higher in negative value than the corresponding virgin activated carbon [23] [35] . The zeta potential increases as the in initial concentration of cationic surfactant increases but remains constant above the critical micelle concentration of each surfactant [22] .

Xu et al. [20] observed that perchlorate removed by virgin or CTACmodified activated carbon peaked at pH 2 - 3 corresponding to their pH_pzc (pH 2 for virgin activated carbon; pH 3 for CTAC modified activated carbon). Below their pH_pzc perchlorate specific uptake decreased as the pH decreased and above their pH_pzc perchlorate specific uptake decreased as the pH increased. At every pH percentage removed by CTACmodified activated carbon consistently greater than virgin activated carbon.

Tang et al. [21] obtained pH_pzc 4.3 and 4.9 respectively for virgin activated carbon and CTAC modified activated carbon. The zeta potential of the CTACmodified activated carbon consistently higher in positive value at every pH indicating CTAC modified activated carbon has more positive surface charge than the virgin activated carbon. Within the pH 3 - 11 range studied, specific perchlorate uptake by the virgin activated carbon decreased moderately from 9.8 mg/g at pH 3 to ~7.8 mg/g at pH 11. The specific perchlorate uptake by the CTAC modified activated carbon decreased rapidly from 27.31 mg/g at pH 3 to 10.58 mg/g at pH 11. Of note is that at each pH studied specific perchlorate uptake by CTAC modified activated carbon consistently greater than the virgin activated carbon. Other researchers obtained similar results [27] [28] [29] .

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 pH_pzc, 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 pH_pzc reduced specific uptake of nitrate by CTAB modified activated carbon [28] and perchlorate by CTAC modified activated carbon [20] possibly due to repulsion between the protonated anions and positively charged amines groups on the cationic modified activated carbon [36] .

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. For instance the specific Cd (II) uptake by DSS modified activated carbon, SDBS modified activated carbonand SDS modified activated carbonrespectively at pH 2 were 0.065, 0.075, and 0.09 mmol/g but increased steadily to 0.13, 0.140 and 0.200 mmol/g at pH 6 [23] . Of note is that the uptake by each of the anionic modified activated carbonswas greater than the virgin activated carbon at every pH. This conforms to the observation of Kakavandi et al. [26] regarding the adsorption of Co (II) by virgin and SDSmodified activated carbon.

This behaviour was attributed partly to reduction in competition between H⁺ and Cd (II) for binding on the same sites as pH increases [37] [38] . In addition, increase in the net negative surface charge of the anionic modified surfactant as the pH increases causes attraction between the targeted cation and the negatively charged surface. However, excessive high pH should be avoided to prevent precipitation of metal hydroxide [23] [26] . The danger of microprecipitation starts at pH > 5 for lead and at pH 6.7 for nickel [39] . Microprecipitation starts at pH > 6 for copper [40] [41] , at pH 7 for chromium [42] and at pH > 8 for cadium [43] . Formation of such insoluble precipitate may introduce errors into adsorption results as the removal of metal ions in the presence of such precipitate is not completely through binding on the surface function groups [44] . Occurrence of desorption of anionic surfactant anchored on the activated carbon is also high at a high pH [23] .

Percentage inorganic contaminants removed by adsorbents usually decreases as initial ion concentration increases [26] [28] [45] [46] [47] . At low concentration of contaminant the available active binding sites are more than enough while at high initial concentration the binding sites are not enough; hence the percentage adsorbed will be higher at low initial concentration due to abundant available active bind sites compared to the number of targeted ions than at high initial concentration.

In contrast, adsorption capacity usually increases with increase in initial concentration [26] [28] [47] [48] [49] . This trend is the same for both cation adsorbed onto anionic surfactant modified activated carbon and anion adsorbed onto cationic surfactant modified activated carbon. For instance, Kakavandi et al. [26] observed that at pH 7, 1.2 g/L adsorbent concentration, 20˚C, 200 rpm, Co (II) specific uptake by SDS modified activated carbon increased from 16.4 to 40.8 mg/g while the percentage cation removed decreased from 98.2% to 49% as the initial Co (II) concentration increased from 20 to 100 mg/L. Similarly, Allalou et al. [28] recorded an increase in the specific nitrate uptake from 32 to 80 mg/g but decrease in percentage anion removed from 92% to 40% as the initial concentration increased from 100 to 600 mg/L.

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 [28] . This will make the ratio of adsorbed concentration to initial concentration decreases as the initial adsorbate concentration increases [16] . However, increase in initial concentration increases the driving force causing more adsorbate to bind onto the adsorbent. Hence, for a fixed mass of adsorbent, increase in initial concentration of adsorbate increases the specific adsorbate uptake until equilibrium is attained [26] [27] .

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. [26] at constant Co (II) initial concentration, recorded an increase in Co (II) removed from 51.3% to 95% but decrease in the specific ion uptake from 38.5 to 19 mg/g as SDS modified activated carbon concentration increased from 0.4 to 1.5 g/L. Similarly, Mazarji et al., [27] at constant 40 mg/L initial nitrate concentration obtained increase in percentage nitrate removed from 25% to 83% but a decrease in specific uptake from 11 to 3.3 mg/g as CTAB modified activated carbon concentration increased from 0.05 to 0.5 g/L.

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 [50] [51] . Reduction in the specific ion uptake with increase in adsorbent concentration could be due to unsaturation and/or lower exploitation of the active binding sites at high adsorbent concentration. In addition, at high adsorbent concentration particles are self-bound together and form aggregate; hence reduces the effective active binding sites available for adsorption. Therefore, the specific ion adsorbed reduces with increase in the adsorbent concentration [52] [53] [54] .

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 [22] , reduce rate [25] [29] or maintain rate [28] when compared to virgin activated carbons.

Specifically, Farooq et al. [22] observed a decreasing order of adsorption rate of bromate onto various adsorbents as CTAC modified activated carbon > CPC modified activated carbon > CTAB modified activated carbon ≈ virgin activated carbon. This suggests that bromate, Bro 3 – exchange may occur rapidly with chloride counter ions, Cl – on CTAC and CPC than with bromide counter ion, Br – on the CTAB. Besides, increase in temperature of endothermic adsorption causes increase in rate of adsorption. For instance, pseudo second order rate constant of CTAC modified activated carbon increased from 0.0125 to 0.0311 g/(mg∙min) as the solution temperature increased from 15˚C to 60˚C [21] .

In contrast, Chen et al. [25] observed a decreasing adsorption rate after surfactant modification with virgin granular activated carbon adsorption rate > CTAC modified activated carbon. Some researchers also obtained reduction in adsorption rate after modification with polymer [29] . Of note is that surfactant modification usually reduces the BET specific surface area of the modified activated carbon due to micropore blockage. Hence surface area and micro pore volume obtained by different authors after modification are different. In addition, the process parameters such solution temperature, solution pH and agitation rates used by different author differ. All these could contribute to different trends of adsorption rate observed by different authors.

Equilibrum times reported by different authors also differ. Allalou et al. [28] observed a rapid increase in percentage nitrate removed in the first 5 min contact. Their system attained equilibrium within 5 min of contact for both virgin activated carbon and the CTAB modified activated carbon. Similarly, specific perchlorate uptake adsorbed onto CTAC modified activated carbon increased rapidly in the first 15 min of contact accounting for 90% of the total perchlorate uptake but the equilibrium was attained in 30 min [21] .

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 Table 1. For instance, Chen et al. [25] system attained equilibrium around 360 min but they used 1440 min as contact time for isotherm studies perhaps to ensure the maximum uptake that can be obtained for the modified activated carbon. In practice, rapid adsorption is most desired to allow a substantial reduction in the size of the adsorption column. This will make the process more cost effective.

Many authors conducted adsorption of inorganic contaminant onto surfactant modified activated carbon at ambient temperature [17] [23] [25] [28] [29] . However, few studies show that temperature has moderate effect on the percentage contaminant removed or specific contaminant uptake by the modified activated carbons indicating the process is either endothermic or exothermic.

Tang et al. [21] observed that the specific perchlorate uptake by CTAC modified activated carbon increased from 35.5 to 55.3 mg/g as temperature increased from 25˚C to 60˚C. They obtained positive change in enthalpy (∆H) implying the adsorption is endothermic. Negative Gibb free energy (∆G) was obtained indicating the adsorption is spontaneous. A positive entropy (∆S), 22 J/(mol∙K) was obtained implying a net increase in randomness of perchlorate at the solid/solution interface occurred in the internal structure of the adsorbent.

Cho et al. [29] recorded decrease in specific uptake from 29.4 and 77.2 to 24.4 and 56.6 mg/g respectively for both nitrate and Cr (VI) as temperature increased from 25˚C to 45˚C. In addition, negative change in enthalpy (∆H) and change in Gribb free energy (∆G) were obtained for both ions indicating the process is exothermic and spontaneous. They also obtained a positive change in entropy (∆S). Hence, operating such exothermic adsorption at ambient will save cost if this does not significantly reduce the adsorption rate.

Increase in ionic strength (NaCl concentration) suppresses ion exchange but enhances non-electrostatic (hydrophobic) attraction [55] . Hence, the net uptake of inorganic contaminant by surfactant modified activated carbons in the presence of NaCl depends on the dominant adsorption mechanism. For instance, Tang et al. [21] recorded a rapid decrease in specific perchlorate uptake by CTAC modified activated carbon from 24.8 to 18.4 mg/g as the NaCl concentration increased from 0 to 10 mmol/L. Further increase in NaCl concentration to 50 mmol/L caused slight reduction to 18.06 mg/g.

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 [55] [56] . In addition, competition between chloride ion, Cl – and perchlorate ions, ClO 4 − for the same active binding sites on the adsorbent also contribute to the reduction in perchlorate adsorption with increase in ionic strength [56] .

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. [20] in the first 60 min of contact obtained increase in specific perchlorate uptake from 1.34 to 1.80 mg/g as the ionic strength increased from 0 to 1.71 mmol/L. Further increase in ionic strength to 8.55 mmol/L decreased the specific uptake to 1.54 mg/g. The inhibition became significant at 120 min contact making the adsorption rate decreased from 46.35 g/(mmol∙h) with no NaCl addition to 27.2 and 18.3 g/(mmol∙h) respectively with addition of 1.71 and 8.55 mmol/L NaCl. Pham et al. [57] observed similar results when using SDS modified laterite to adsorb Copper (II) ion. They recorded increase in Cu²⁺ uptake from 150 to 185 mg/g as the ionic strength increased from 1 to 10 mmol/L. But further increase to 50 mmol/L NaCl caused reduction of uptake to 110 mg/g. These results also suggest that electrostatic and non-electrostatic (hydrophobic) attraction mechanisms are occurring during adsorption of inorganic contaminants by surfactant modified activated carbon.

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 [58] . Chen et al. [25] added a varied (0.049 - 159.2 mg/L) of perchlorate, nitrate, sulphate or phosphate ions to a fixed bromate concentration (0.1 mg/L) to study its effect on the specific bromate uptake by CTAC modified activated carbon. The competing co-ions’ inhibitory effect is insignificant at 10:1 co-ions to bromate molar ratio. This could be due to abundant active binding site at such low concentration. However, perchlorate and nitrate’s inhibition became significant at 100:1 co-ions to bromate molar ratio while sulphate and phosphate’s inhibition became significant at 1000:1 co-ions to bromate molar ratio. At the same co-ions to bromate molar ratio, competition followed decreasing order of ClO 4 − > NO 3 − > SO 4 2 − > PO 4 3 − . This order suggests that competition decreases as valence electrons increase. Other researcher observed similar results [59] .

Mazarji et al. [27] also studied the effect of adding each of phosphate, carbonate, sulphate and chloride on the specific nitrate uptake by NaOH-CTAB modified activated carbon. The co-ions concentrations were varied from 20 to 160 mg/L while nitrate was fixed at 40 mg/L. Addition of each of the co-ions significantly reduced the specific nitrate uptake. Competition followed a decreasing order of SO 4 2 − > Cl − > PO 4 3 − > CO 3 2 − . The least effect of phosphate and carbonate was attributed to formation of H 2 CO 3 and H 2 PO 4 − species which have less degree of competing with nitrate for active binding sites.

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 [21] [22] while other mechanism such electrostatic attraction, surface complexation and physical adsorption do complete it [23] [25] . Hence, if anion is the targeted contaminant to be removed from the aqueous solution, proper cationic surfactant is required for the modification of the activated carbon [17] [21] [22] [27] [28] [29] . In contrast, if cation is the targeted contaminant, then proper anionic surfactant should be selected for the modification [23] [24] [26] . The flexibility in anchoring of surfactant onto the surface of activated carbon makes it one of the appealing surface modification methods.

The surfactant concentration used for modification of activated carbon also contributes to its adsorption capacity. Chen et al. [25] varied CTAC concentration from 0.2 to 8 mmol/L to obtain the proper surfactant concentration that maximise the uptake capacity of the modified activated carbon. The specific bromate uptake peaked at 2 mmol/L, equivalent to 1.26*critical micelle concentration of CTAC, below which the specific uptake steadily reduced and above which the specific uptake equally reduced.

Similarly, Kakavandi et al. [26] varied the SDS concentration from 0.5 to 5*CMC in order to optimise the specific Co (II) uptake by SDS modified activated carbon. The Co (II) removed peaked at 1.5*CMC of SDS, below and above which the specific uptake reduced steadily reaching the least value, 0% at 5*CMC. Of note is that at 6 h (optimised time) the specific uptake of the modified activated carbon for all concentration except 5*CMC were greater than that produced by virgin activated carbon.

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 [22] while the surface area reduce [22] [24] [27] [28] due to surfactant blockage of micropores [24] [27] . This explains the reduction in specific ion uptake observed by Chen et al. [25] and Kakavandi et al. [26] with excessive increase in surfactant concentration above 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 [17] [27] or above CMC [21] during adsorption experiments. Such approach may prevent optimising the capacity of the modified activated carbons.

Mazarji et al. [27] observed that treatment of activated carbon with NaOH at 75˚C for 7 h prior to CTAB surfactant modification enhances its adsorption for nitrate. They obtained percentage nitrate removed as NaOH-CTAB modified activated carbon, 80% > NaOH treated activated carbon, 16% > virgin activated carbon, 8%. The observed enhancement was attributed to the increase in the specific surface area and micro pore volume caused by NaOH treatment. More micro pores allowed anchoring of more CTAB thereby increasing the net positive surface charge [60] . 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] .

Sun et al. [24] treated activated carbon with HNO₃ before modification with sodium dodecyl benzene sulfonate (SDBS) to optimise its Cd (II) adsorption. HNO₃ concentration was varied from 0.5 - 4 mol/L, temperature, 60˚C - 100˚C, and SDBS, 5000 - 40,000 mg/L. The specific Cd (II) uptake by the HNO₃-SDBS modified activated carbon peaked at 3.29 mol/L HNO₃, 76˚C and 30, 700 mg/L SDBS. They observed that interaction of HNO₃ and surfactant concentration produced most significant effect on the maximum Cd (II) uptake capacity. The specific Cd (II) uptake capacity order obtained are HNO₃-SDBS modified activated carbon, 44.2 mg/g > SDBS modified activated carbon, 26.60 mg/g > HNO₃ modified activated carbon, 22.78 mg/g > virgin activated carbon, 6.78 mg/g. Of note is that the developed surface area, functional groups, and more net negative surface charge responsible for the higher adsorption capacity show by the HNO3-SDBS modified activated carbon.