The point of zero charges (pHpzc) indicates a pH value of zero. This parameter indicates the interface between the positive and negative charges at the active sites. A potentiometric titration involving 0.001 M, 0.01 M, and 0.1 M of NaCl electrolyte, and 1 g activated carbon reacted for 24 h was used to evaluate this parameter. The titration curves generated by a plot of net proton charges versus the suspension pH were used to determine the pHpzc.

Morphological study of the activated carbons

The scanning electron microscopy (SEM–EDX method) (EMLS-Canada) was used to perform the image analysis of the activated carbons. A high vacuum mode (HV) was used to observe the analyzed samples at the accelerated voltage of 20 kV or 10 kV using an SE secondary electron detector.

Determination of surface area of activated carbon

The physical adsorption of nitrogen gas on the surface of the AC was used to evaluate the equivalent nitrogen gas adsorbed to a single layer on the activated carbon surface. monomolecular layer on the surface. A micromeritics (ASAP 2020) surface area analyzer LabX, Ontario, Canada was used to degas the sample at 300 °C in liquid nitrogen flow condition (Stoeckli and Centeno, 2005).

Regeneration of the activated carbon

Thermal regeneration of the spent activated carbon was carried out by treating it with 1 M nitric acid, thus resulting to 60% regain. The extracted activated carbon was heated to 500 °C for 24 h (Shahadat, and Isamil 2018).

Adsorption experiments

All chemicals and materials used for analyses were of analytical grade purity obtained from BDH (England), Loba Chemie (India). 1 g of mercury chloride was dissolved in distilled and made up to a liter. This content is the stock solution, and several standards were prepared from the stock solution. The equilibrated experiments on batch mode adsorption were administered using 20 g/50 mL of mercury solution. The suspension since agitation for 30 min except for contact time experiments and filtered with 0.45 µm membrane was measured for Hg(II) ions remaining in a solution using ICP-OES analyzer (CAP6300, Thermo, USA). The adsorption capacity of the AC Qe (mg/g) was determined (Eq. 1):

The adsorption capacity of the AC Qe (mg/g) was calculated using the following Eq. (1):

Herein, C0 and Ce define the concentration at the start of reaction and concentration at the steady- state of the Hg(II) ions (mg/L) and contaminated water interaction, respectively, V is the volume of the solution (L), and m is the mass of AC used (g). The exchange of protons has a constant defined by an isotherm deduced from the plot of the change in pH versus lg Kd as provided (Eqs. (5) and (6):

Herein, SOH represents an AC surface-reactive site, SO − is the surface-bound Hg(II), lgKp is the apparent equilibrium-binding constant, and α is the coefficient of protonation, which represents the number of protons displaced when one mole of Hg(II) binds to the AC surface.

Also, the mass transfer rate and intraparticle diffusion were derived from Eqs. (8)– (9):

Herein, C₀ is the initial Hg(II)concentration (mg·L⁻¹) at time t = 0; C_t is the concentration (mg·L⁻¹) at time t; V is the total suspension volume, and m is the mass of the AC (g) (Ojemaye et al. 2017).

An intraparticle diffusion model by Weber–Morris also, was used to describe the reaction mechanism as provided (Eq. 8) (Dong, 2012):

A reaction kinetic involving AC charged with Hg(II) ions was deduced from the mass transfer constant K_f. Here, C_t/C₀ versus time provided the slopes of the curves derived from Eq. (8) {Joseph et al. 2019}.:

Herein, C₀ and C_t represent the Hg(II) initial concentration at time t, S_s is the exposed surface area of an AC, and K_f is the coefficient of mass transfer. These models as derived from the Freundlich isotherm have been reviewed previously and adopted to describe the adsorption of Hg(II) ions(Mohan and Chander 2001).

Herein, K_i is the intraparticle diffusion constant (mg·g⁻¹·min⁻¹), and the intercept (C) represents the effect of the layer boundary. The value of K_i is derived from the slope (K_i) of the plots of Q_t vs. t^0.5. A linear plot of Q_t versus t^0.5 indicates that intraparticle diffusion was involved in the process of adsorption [Li et al. 2016]. For validation of an intraparticle diffusion, a 10 mg·L⁻¹ Hg(II) solution was charged with 20 g of AC made up to 50 mL at 4 ≤ pH ≤ 10. This mixture was treated for 30, 60, 90, 120, 150, 180, and 210 min. A Hg(II) ions remaining in solution were determined using the ICP-OES. In all experiments, the adsorption efficiency of AC was calculated from Eq. (10):

Herein, C₀ and C_e (mg·L⁻¹) are the concentrations at the start of reaction and concentration at the steady-state Hg(II) concentrations in solution, respectively (Zbair et al. 2019; Awual, 2017).

Besides the Freundlich isotherm and its derivatives used in this study, the Langmuir adsorption isotherm is used to describe the adsorption kinetics. A linear regression (y = A + B · x) and Langmuir equation are represented

Herein, C_e is the equilibrium concentration of cadmium(mg/L) in solution, q_e is the adsorption capacity of the activated carbon at equilibrium (mg/g), q_m is the maximum adsorption capacity of the activated carbon (mg/g) and b(L/mg) is a constant linked to the energy of adsorption. A plot of ��/�� versus �� gives a slope of 1�� and an intercept 1��∗�.

y = A + B*x; y = ��/��; x = ��; A = 1��∗�; B = 1�� (12).

If the adsorption process falls in line with the Langmuir model, then the plot provides a straight line, and the values of q_m and b (L/mg) are evaluated from the plot.

Effect of pH on the removal of Hg(II) removal

At a constant contact time (30 min), dosage (20 g), and some mercury concentration (20 mg/L) the effect of pH was also studied. 20 g of the activated carbon was introduced into a 50 mL plastic vial. The content was made up to 50 mL by adding the mercury-contaminated carnal water and 0.1 M of NaOH was used to regulate the pH to pH 4, 5, 6, 7, 8, 9, and 10. The solution was shaken and filtered with 0.45 µm filter paper. The Remaining mercury in solution was determined using inductively coupled plasma optical emission spectrometer (ICP-OES analyzer CAP6300, Thermo, USA).

Effect of contact time on the removal of Hg (II) Removal

At a constant pH of 6.05, 20 g dosage and some mercury concentration of 20 mg/L, the effect of contact time on mercury removal was also studied at a range of 30, 60, 90, 120, 150, 180, and 210 min. 20 g of the activated carbon was introduced into a 50 mL plastic vial. The content was sealed and placed in a mechanical shaker and agitated at 20 rotation per minute (rpm), for each of the different contact times (30, 60, 90, 120, 150, 180, and 210 min). The content of each was filtered and analyzed with ICP-OES analyzer (CAP6300, Thermo, USA) to determine the amount of mercury remaining in the solutions.

Effects of Hg concentration on its removal by activated carbon

At a constant pH of 6.05, a contact time of 30 min and 20 g dosage of activated carbon, the effect of Hg concentration on its removal by activated carbon was studied. 20 g of the activated carbon was introduced into a 50 mL plastic vial. The content was made up to 50 mL by adding variable concentrations of mercury solutions at 40 mg/L, 20 mg/L, 15 mg/L, and 10 mg/L. The content was sealed and placed in a mechanical shaker and agitated at 20 rotations per minute (rpm). The content was filtered and analyzed with ICP-OES analyzer (CAP6300, Thermo, USA) to determine the amount of mercury remaining in the solutions.

The effect of dosage of activated carbon on the removal of Hg(II) ions

At a constant pH of 6.05, a contact time of 30 min a mercury concentration of 20 mg/L, the effect of dosage of activated carbon on mercury removal was studied. 10, 20, 30, and 40 g of the activated carbon were introduced into a 50 mL sample vial. The content was made up to 50 mL by adding 20 mg/L of mercury solution. The content was sealed and agitated in a mechanical shaker at a speed of 20 rotations per minute (rpm). The content was then filtered and analyzed with ICP-OES analyzer (CAP6300, Thermo, USA) to determine the amount of mercury remaining in the solutions.