A stock mercury ion solution (1,000 mg/L) was prepared by dissolving analytical grade mercuric chloride (HgCl₂) in double distilled water. The desired pH values of the working solutions were adjusted using 0.1 N NaOH and 0.1 N HCl. The mercury concentration was determined using atomic absorption spectrophotometer (Shimadzu AAS 6200).

Tridax procumbens (Asteraceae) is an important medicinal plant widely distributed in agricultural fields. The plant leaves were collected and air dried for 48 hr. The dried leaves were grounded in ball mills and the screened homogeneous powder was used for the preparation of biocarbon. The activated biocarbon was prepared by treating the leaves powder with the concentrated sulphuric acid (Sp. gr.1.84) in a weight ratio of 1 : 1.8 (biomaterial : acid). The resulting black product was kept in an air-free oven, maintained at 160 ± 5°C for 6 hr followed by washing with distilled water until free of excess acid, then dried at 105 ± 5°C. The particle size of activated carbon between 100 and 120 μm was used [15].

Biosorption studies were conducted in desired number of 250 mL Erlenmeyer flask containing a 100 mL of metal ion solution. The effect of pH, contact time, amount of biocarbon dose and influence of temperature (30 to 60°C) were evaluated by using the initial metal ion concentration of 100 mg/L. For optimization of the amount of biosorbent; 0.5, 1.0, 1.5, 2.0, 2.5, 3.0 and 3.5 g of biocarbon was treated with the test solution (pH 4.0). For the comparative evaluation, metal ion concentration of 25 and 50 mg/L were carried out at the optimum pH of 5.5. All the experiments, expect influence of temperature on biosorption of metal ion was carried out at 28 ± 2°C. In the batch experiments, flasks were agitated at 250 rpm constantly up to 160 min to establish the equilibrium condition and samples were collected at the end of the pre-determined time (20 min) intervals and the amount of metal ions content was analyzed using cold vapour AAS technique. The pH of the test solution was monitored by using a Hanna pH Instruments (Italy).

Biosorption values were determined as the difference between the initial metal concentration and that in the supernatant. The amount of metal ion adsorbed per unit mass of the biosorbent was evaluated by using the following equation:

where q_e = amount of metal ion adsorbed (mg/g), V = volume of metal ion solution (mL), w = mass of biocarbon (g), C_o = initial metal ion concentration (mg/L) and C_e = metal ion concentration at equilibrium (mg/L). The percent of metal ion removal was evaluated from the equation:

The characteristics of the biocarbon reveal that, total carbon content was high which has a more carbonaceous matter and has less ash content (Table 1). Bulk density and particle size of biocarbon are essential parameters which should be known before its application in wastewater treatment systems. It is observed that, the surface is relatively high and is responsible for potential removal of metal ions from synthetic wastewater system. Further, the values of methylene blue, ion exchange capacity and phenol number are also significantly good. The result indicates the biocarbon with good activity for adsorption.

The Fourier-transform infrared spectroscopy (FT-IR) technique is an important tool to identify the characteristic functional groups present on the biocarbon surface (Fig. 1). The FT-IR spectrum was recorded on a Spectrum One (Perkin Elmer, USA). The important peaks of infra-red analysis are shown in Table 2. The FT-IR analyses support that the main mechanism of biosorption of mercury was chelating with different functional groups such as –OH, –C=O, –C=C, and –C–H. The presence of polar groups on the biocarbon surface is likely to be responsible for the metal ion removal from the bulk solution.

In addition, scanning electron microscopy (SEM) was used to visualize the surface morphology of the biocarbon (Fig. 2). The sample was scanned by using JSM 6701F (JEOL, Japan). The SEM micrograph of biocarbon indicates the regular morphological structure of the particles and also presents some leaf like structures and large numbers of nano grains which makes possible for the adsorption of heavy metal ions on different parts of the biocarbon.

The pH of the solution is one of the important factors governing the biosorption of metal ions. The removal of Hg(II) ions was performed with the initial concentration of 100 mg/L and the optimized biocarbon dose of 2.5 g. Fig. 3 shows the effect of pH change in the range 2 to 8 with the interval of 0.5 on the biosorption of Hg(II) on the biocarbon. The initial pH of the solution has a significant influence on the amount of metal ion sorbed. It was observed that removal Hg(II) ions in the aqueous solution is low at low pH values and increased with increasing pH of the solution. The optimum pH for the maximum removal (96.5 %) of Hg(II) was 5.5. Further increase of pH leads to decrease in Hg(II) removal efficiency. The influence of pH can be related with electrostatic repulsion where its decrease will be associated with reduction of positive charge density on the sorption sites thus resulting in an enhancement of metal biosorption [16].

The effect of contact time on Hg(II) ions removal was investigated at regular time intervals of 20 minutes at an initial concentration of 100 mg/L and the profiles of metal ion biosorption trends and experimental conditions was shown in Fig. 4. The biosorption was progressed steadily and reached to the equilibrium at 160 min. After the equilibrium is reached, the biosorption of metal ions did not change significantly. The maximum biosorption of metal ion may be due to the presence of large number of accessible binding sites and the affinity of the functional groups on the biocarbon, as well as the mode of interaction between the metal ion and the biocarbon [17].

The initial mercury (II) ions concentration of the solution is an important factor which influences the aqueous phase equilibrium between adsorbed species and biosorbent. Generally, biosorption decreases with increasing metal ions strength of the aqueous solution. This is explained by the fact that at low concentrations, most of the metal ions are sorbed by specific active sites and at higher concentrations the biosorption of metal ions decreases significantly. The phenomenon can further be attributed to the saturation of sorption sites on the biocarbon [18]. A result demonstrating the effect of initial concentration on removal of Hg(II) was shown in Fig. 5. The biosorption of metal ions increases from 92.5% to 97.0% as the concentration increases from 25 to 50 mg/L. This observation is attributed to the increase in the driving force which overcomes all mass transfer resistance of metal ion between the aqueous and solid phases [19]. The established equilibrium time is 60 and 100 min respectively. Similar results were also reported by researchers who indicated that increasing initial mercury concentrations in solution resulted in increasing its biosorption efficiency until the saturation of adsorbing sites are reached [20–22].

The effect of biocarbon dosage on the biosorption of Hg(II) ions was performed with the initial metal ion concentration of 100 mg/L at the pH of 5.5 as shown in Fig. 6. The biosorption efficiency of the biosorbent gradually increases as the amount of biocarbon is varied from 0.5 to 3.5 g. It was observed that, the maximum (96.5%) removal of Hg(II) was established at 160 min at the biocarbon dose of 2.5 g. This can be attributed to the fact that, an increase in biosorbent dosage increases the surface area of biosorbent leading to an increase in the number of metal binding site on the biosorbent surface. This factor also reveals the presences of large number of vacant binding sites, which are available for biosorption during the initial stages, but thereafter the attraction to the remaining vacant sites will be difficult due to the repulsive forces between the Hg(II) ions on the biosorbent and the liquid phases [8, 23].

The removal of Hg(II) ions was evaluated at four temperatures as 30, 40, 50 and 60°C. The initial concentration of metal ions was 100 mg/L and pH was 5.5. The results indicate that, the highest removal was obtained in the temperature 40°C and then decreased with the increase of temperature (Fig. 7). It indicates the process to be endothermic. Hence, rise in temperature beyond certain limits might causes a decrease in sorption capacity of the biocarbon.

The study of application of biosorption isotherm models to the equilibrium data is important to optimize the design of the biosorption system for the removal of the metal ions. The biosorption of a substance from one phase to another leads to a thermodynamically defined distribution of that substance between the phases as the system reaches equilibrium state [24]. The most common biosorption isotherm models of Langmuir and Freundlich have been tested in the present study.

The data obtained from biosorption of mercury (II) was analyzed using Langmuir isotherms. The linear form of the Langmuir isotherm is given by the following equation.

where, q_e is the equilibrium metal ion concentration on the biosorbent (mg/g), q_max is the maximum biosorption capacity of biosorbent (mg/g) and represents a practical limiting biosorption capacity when the surface is fully covered with metal ions. C_e is the equilibrium concentration of the metal in solution (mg/L). The plot of C_e versus C_e/q_e should give a straight line, its slope equals to 1/q_max and the intercept has the value of 1/(q_max b), where b is the biosorption coefficient (L/mg). From the plot (Fig. 8(a)), q_max and b values of the Hg(II) were calculated and given in the Table 3.

The essential characteristics of Langmuir isotherm can be expressed by a dimensionless constant called equilibrium parameter RL, defined by Amin [25].

where C₀ is the initial concentration of metal ion (mg/L). The characteristics of the R_L value indicates the nature of biosorption as unfavourable (R_L > 1), linear (R_L = 1), favourable (0 < R_L < 1) and irreversible (R_L = 0). The obtained value of R_L is 0.0321 at 28°C which shows that the biosorption of Hg(II) ions onto the biocarbon is favorable.

The Freundlich biosorption isotherm model is an empirical equation based on the biosorption on a heterogeneous surface [26]. The linear form of Freundlich biosorption isotherm can be defined by the following equation:

The constant n is an empirical parameter which reflects the intensity of biosorption that varies with the degree of heterogeneity and K_f is a constant related to biosorption capacity. The constants n and K_f were calculated by plotting log C_e against log q_e (the slope = 1/n and the intercept = log K_f). The results were illustrated in Fig. 8(b). The values of n between 1 and 10 represent a favorable biosorption. The obtained value of n is 2.34 shows that the biosorption is more favorable.

The kinetics of biosorption of Hg(II) ions was studied by using the two important kinetic models of Lagergren pseudo-first-order and pseudo-second-order equations. The general pseudo-first-order equation expressed as

where q_e and q_t are the amounts of metal ions sorbed onto the biocarbon (mg/g) at equilibrium and at time t (min) respectively. By plotting the log (q_e – q_t) versus t, the first-order rate constant k₁ (1/min) and the equilibrium capacity q_e can be obtained from the slope and intercept respectively.

The linear form of the pseudo-second-order chemisorption kinetics rate equation is expressed as

The constants can be determined by plotting t/q_t versus t. The second-order rate constant k₂ (g/(mg·min)) and q_e (mg/g) values can be calculated from the intercept and slope of the plot. The results of the pseudo-first-order and second-order kinetics are shown the Fig. 9(a) and 9(b). The rate constants, metal ion uptake and correlation coefficients have been described in Table 4. It was observed that, the kinetics of the biosorption process followed the pseudo-second order model (r² = 0.9940) better than the Lagergren model (r² = 0.8810). The pseudo second order reaction in biosorption is based on the sorption capacity on the solid phase. The correlation coefficient of the pseudo second order reaction was high and this factor suggesting that the biosorption process comply pseudo second order model for the removal of Hg(II) ions.