1 Introduction

Heavy metals are potential water pollutants [1] due to their toxic and non-biodegradable nature that pose a severe threat to public health and the environment [2, 3]. The main sources of metal pollution are tannery, mining [4], metallurgy, electroplating, production of chemicals, painting, battery production, surface modification industry, garbage incinerator, sludge discharge, processing of radioactive materials, production of electric equipment, pesticides and fertilizers [5] and dyes [6]. According to the World Health Organization (WHO), aluminum, chrome, cobalt, nickel, copper, cadmium, zinc, mercury, and lead are considered as toxic metals [7, 8]. The adverse effects of heavy metal ions on humans [9] and other living organisms are continuously increasing from the last of few decades [10, 11]. The WHO reported that about 50,000 people die each day as a result of water pollution [12]. Cadmium (Cd) and lead (Pb) are the heavy metals which are most likely to be found in waste water [13, 14] that may ultimately find their way into the natural water resources [15,16,17]. Cadmium is a non-essential toxic heavy metal which is commonly used in batteries, pigments, corrosion resistant coatings [18], ceramics and textile printing [19]. It is generally released into the environment as a result of burning of fossil fuels, use of phosphate fertilizers, incineration of municipal solid wastes, tobacco smoking, and manufacturing of iron, steel and cement [20]. Similarly, lead is highly toxic metal that adversely affects the living organisms [21]. It is used in construction material, corrosion resistant, electrode material, shield for radiation and automobile batteries. Lead may enter the water bodies through effluents from paints, pigments, batteries, solders, alloying and mining activities [22]. It may enter to the environment by combustion of leaded gasoline in the internal combustion engines [23, 24]. According to World Health Organization (WHO), the permissible limits of cadmium and lead in drinking water are 0.003 mg/L and 0.001 mg/L, respectively [25].

Several studies have revealed that cadmium and lead concentrations in drinking water in Pakistan particularly in urban areas are above the maximum permissible limits of W.H.O.[25]. Due to the toxic nature of cadmium and lead, their removal from potable and waste waters by eco-friendly methods is of utmost importance [26]. Most of the methods used for water treatment are expensive, technically complicated or associated with toxic wastes disposal problems. Adsorption is an effective, simple, environment friendly and cost effective method for the uptake of heavy metal ions from aqueous media [27, 28]. The use of agricultural wastes for preparation of activated carbons is increasing due to their better adsorption capacities and cost effectiveness. Various agricultural wastes like corn cobs, hazelnut shells, date stones and nutshells have been used by researchers for the preparation of activated carbons. Whereas, the agro-carbons of varying surface area, pore volumes and pore size distribution can be prepared by controlling the conditions of carbonization process [29, 30].

The present study focuses on the preparation of surface oxidized and un-oxidized activated carbon from saw dust of Ziziphus jujube (Ber) plants by chemical activation with phosphoric acid (H3PO4). A very little work has been reported in literature regarding the use of Ziziphus jujube as starting material for preparation of activated carbon. Pandharipade et al. (2012) has reported synthesis of adsorbents from waste materials of the Ziziphus jujube seed [31]. As such, a large volume of Ziziphus jujube saw dust is produced by saw mills in Pakistan which is of no specific utilization as useful commodity. The saw dust is discharged and dispersed into the environment without proper disposal management that may cause several potential hazards. The conversion of such agricultural wastes into valuable products like activated carbons for the removal of toxic metal ions from aqueous solutions is of great importance from both environmental and economic point of view. The prepared carbon samples were utilized for the adsorptive removal of Cd(II) and Pb(II) metal ions from aqueous solutions.

2 Materials and methods

2.1 Chemicals and instruments

Analytical grade chemicals, H3PO4 (CAS: 7664–38-2), HCl (No. 109,058), NaOH (CAS: 1310–73-2), Cd(NO3)2 (No. 102,019) and Pb(NO3)2 (CAS: 10099–74-8) were purchazed from Merck, Germany. Water was freshly distilled prior to use.

The pH was measured with pH meter (model 744, Metrohm) equipped with a combined glass-saturated calomel electrode calibrated with buffer solutions of pH ~ 4.0, pH ~ 7.0 and pH ~ 10.0. The nitrogen adsorption isotherms were determined with a Quantachrome NOVA 2200e, surface area and pore size analyzer. The carbon samples were dried in program controller Nebertherm C-19 (model N 7/4 W—Germany). Metal ion concentration was determined using atomic adsorption spectrophotometer (Shimadzu model AA670). The carbonization was carried out in a muffle furnace (model No TF 55030C-1 Thermo electron Corporation, USA) and washing was carried out in a Soxhlet extractor.

2.2 Preparation of activated carbon

Activated carbon was prepared from saw dust of Ziziphus jujube. The saw dust was washed with distilled water and dried at 110 °C for 24 h. The sample was mixed with 50% aqueous solution of H3PO4 (weight ratio; 1:1). The mixture was allowed to soak for 24 h at room temperature. The sample was heated in a horizontal tube furnace at 170 °C for one hour under nitrogen atmosphere with flow rate of 100 mL per minute then the temperature was increased to 450 °C for one hour under the same flow rate of nitrogen. The carbonized sample was cooled to room temperature in an inert atmosphere of nitrogen, and the sample was named, un-oxidized activated carbon (USD). Half of the USD sample was oxidized by using breathing grade air at a flow rate of 100 mL per minute at 450 °C for one hour and cooled in the breathing grade air to room temperature. The sample was named, oxidized activated carbon (OSD). Both the activated carbon samples were rinsed with double distilled water in Soxhlet extractor at 100 °C until the pH of the rinsing water become neutral. The samples were dried at 110 °C for 24 h and cooled in desiccators. The cooled samples were sieved and the desired particle size of 170–400 mesh was obtained [32].

2.3 Characterization of activated carbon

The prepared activated carbon samples were characterized by determination of pH, PZC, surface area, pore volumes and surface functional groups. BET surface area and pore structure were determined by nitrogen adsorption method at 77 K using Quantachrome NOVA 2200e, surface area and pore size analyzer. The micro-pore and total pore volume was determined by BJH method. The meso-pore volume was calculated by subtracting the micro-pore volume from the total pore volume. The acidic and basic surface functional groups of each sample were quantified by Boehm’s titration method [33].

2.4 Adsorption studies

Batch experiments were performed at different pH (2–9), initial concentrations (70–100 mg/L), temperatures (298–328 K), activated carbon dosages (0.1–1.0 g) and contact time (5–700 min). Kinetic study was performed using initial concentration of 70 mg/L and pH ~ 6 at different temperatures (298, 308, 318 and 328 K). In batch experiment, 0.1 g of either USD or OSD was added to 40 mL solution of the desired concentration. The initial pH of the solution was adjusted using 0.10 M HCI / NaOH solution. The solution was shaken at a speed of 110 rpm. After shaking, the final pH of solution was recorded and filtered discarding the first 5 mL fraction of each filtrate. The filtrate was analyzed for residual metal ions concentration by using atomic adsorption spectrophotometer. During contact time and kinetic study, each solution was agitated for pre-determined time intervals and filtered. The amount of each metal ion adsorbed (qe) was calculated by the Eq. 1.

$${\text{q}}_{{\text{e}}} = \frac{{(C_{i} - C_{e} )V}}{W \times 1000}$$
(1)

where Ci and Ce are the initial and equilibrium concentrations (mg/L) of metal ions in solution, V is the volume (L) and W is the amount of adsorbent in grams.

3 Results and discussion

3.1 Characterization

Characteristics of the un-oxidized activated carbon (USD) and oxidized activated carbon (OSD) are listed in Table 1. It is evident from the table that both the activated carbon samples have greater pore volumes along with micro-pores and meso-pores. Above 90% of the total pore volume of each sample comprised of micro-pores. The higher BET and lower BJH surface areas also suggested the micro-porous nature of the carbon samples. The increased BET surface area and pore-volumes could be attributed to strong chemical interaction and diffusion of the activating agent (H3PO4) into the inner structure of the precursor material during soaking and impregnation [34]. The activating agent becomes evaporated along with other volatile matter during carbonization, thereby leaving behind pores [35, 36]. Table 1 shows that pH and PZC of aqueous solution of both samples were in acidic range that might be due to the presence of acidic functional groups dominantly lactonic, phenolic and carboxylic functional groups on their surfaces. The acidic functional groups are likely to be introduced during interaction and destruction of the pore walls of carbon samples by phosphoric acid, resulting in deposition of large number of oxygen containing acidic functional groups. The more acidic surface, higher SBET, SBJH, micro-pore volume, average pore-radius and pore volume of the oxidized sample as compared to un-oxidized sample could be explained in terms of air oxidation. The air oxidation might have introduced more oxygen containing acidic functional groups and facilitated the evolution of volatile matter from the precursor materials during carbonization [29, 37].

Table 1 Characteristic parameters of the OSD and USD activated carbon samples
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3.2 Adsorption studies

3.2.1 Effect of contact time and initial concentration

Contact time study helps to determine the rate of adsorption and equilibrium time. The uptake rate is an important aspect; higher adsorption rate indicates higher affinity of adsorbent towards metal ions, and to reach equilibrium state earlier. Equilibrium time depends on the nature, compactness and other textural properties of the adsorbent. Results of the effect of contact time on the adsorption process are shown in Fig. 1. The results clearly illustrate that the adsorption of both the metal ions increased with the contact time until equilibrium was established. The USD indicated the equilibrium time of 480 min for both cadmium and lead while the OSD showed the equilibrium time of 420 min for both metal ions. The adsorption of the metal ions was very fast in the beginning which then becomes slow and finally constant after equilibrium. The higher adsorption in the early stages might be due to the larger number of available sites on the adsorbent surface. The subsequent slower adsorption is likely due to the competition among the metal ions for the limited number of unsaturated adsorption sites. When the adsorbent surface becomes saturated, the diffusion of the metal ions into the bulk of the adsorbent becomes slower and the adsorption rate decreases.

Fig. 1
figure 1

Effect of contact time and initial concentration on Cd(II) and Pb(II) adsorption (pH 6; T = 298 K; Carbon Dose = 0.1 g, Shaking Speed = 110 rpm)

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Furthermore, the results indicate that the sorption capacity of the adsorbent increased with the increase of initial concentration, as shown in Fig. 1. This could be attributed to the increased driving force that minimizes the resistance to mass transfer of metal ions in solution towards the adsorbent surface. At lower concentrations, the number of metal ions available in the solution is less as compared to the sufficiently available active sites on the adsorbent surface, hence rate of adsorption is higher but the amount adsorbed per unit mass of adsorbent is lesser. The increased adsorption of metal ions from solution of higher initial concentrations might be due to covering of all the active sites of adsorbent surface by the sufficient number of metal ions available in the solution.

3.2.2 Effect of pH

pH of the solution is one of the most important parameters, which strongly influences the removal of metal ions from their aqueous solution. It affects the surface charge of adsorbent as well as the solubility and ionic state of metal ion. Activated carbon is amphoteric in nature and at lower pH below pHPZC, its surface becomes positively charged due to high protonation while at higher pH above pHPZC, its surface bears negative charge due to higher concentration of OH−1 ions than H+1 ions. As a result of increased electrostatic attraction, higher adsorption of metal anions occurs at lower pH while favorable adsorption of metal cations takes place at higher pH [38].

The effect of pH on adsorption of Cd(II) and Pb(II) ions was studied in the pH range of 2–9 as illustrated in Fig. 2. As expected the adsorption of both the metal ions increased with increase in initial pH of the solution. The lower adsorption at lower pH could be explained in terms of repulsion between metal cations and positively charged surface of activated carbon. The increased adsorption at higher pH might be due to increased electrostatic attraction between metal cations and negatively charged surface of activated carbon. Indeed the final pH of solution after each metal ions adsorption was found lower than the initial pH which indicated the release of H+1 from the adsorbent surface. The decrease in final pH also suggested that the adsorption of Cd(II) and Pb(II) ions involved an ion-exchange mechanism. As such, OSD indicated lower uptake of metal ions as compared to USD. This might be explained in terms of more acidic surfaces of oxidized samples due to the presence of oxygen containing acidic groups. Higher adsorption of Pb(II) ions occurred than Cd(II) ions under the same conditions, most likely due to smaller size of Pb(II) ions. Most of the metals precipitate at higher pHs, hence to avoid any precipitation of metal ions and being closer to pH of natural water, pH 6 was selected as the optimum pH for the adsorption of Cd(II) and Pb(II) ions from aqueous solutions and all the batch experiments were performed at pH ~ 6.

Fig. 2
figure 2

Effect of pH on Cd(II) and Pb(II) adsorption (Initial Conc: = 70 mg/L; T = 298 K; Carbon Dose = 0.1 g; Shaking Speed = 110 rpm)

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3.2.3 Effect of temperature

The adsorption of metal ions from aqueous solution is highly temperature dependent. Temperature affects the adsorption capacity of porous adsorbent by changing the number of active sites and/or chemical interaction between metal ions and the adsorbent surface. Therefore, the effect of temperature was evaluated at four different temperatures (298, 308, 318 and 328 K) at pH ~ 6 and initial concentration of 70 mg/L.

The results showed that the adsorption of Cd(II) and Pb(II) ions increased with the increase in temperature, as shown in Fig. 3. The enhanced removal at higher temperatures indicated the endothermic nature of adsorption process. The temperature rise might be resulted in creation of some new active sites on adsorbent surface or enhanced interaction between metal ions and adsorbent, leading to increased adsorption. Additionally an increase in temperature decreases the viscosity of the solution due to which the metal ions become more mobile leading to increased diffusion through an external boundary layer and into the pores of the adsorbent surface. At each temperature, USD sample exhibited higher adsorption capacity for both the metal ions.

Fig. 3
figure 3

Effect of temperature on Cd(II) and Pb(II) adsorption (pH 6; Initial Conc = 70 mg/g; Carbon Dose = 0.1 g; Shaking Speed = 110 rpm)

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3.2.4 Effect of carbon dosage

The effect of carbon dosage on removal of Cd(II) and Pb(II) ions was studied in a batch system using carbon dose of 0.1– 1.0 g. Whereas, the initial metal ion concentration, pH and temperature was chose as 70 mg/L, 6 and 298 K, respectively. The results illustrated that the adsorption of Cd(II) ions decreased from 16.47–3.08 mg/g and 14.35–2.36 mg/g while that of Pb(II) ions decreased from 20.98–5.28 mg/g and 18.56–4.19 mg/g respectively by USD and OSD samples when carbon dosage increased from 0.1 to 1.0 g. The decrease in the amounts of Cd(II) and Pb(II) ions adsorbed by both the activated carbon samples might be due to aggregation of adsorbent particles because of the presence of larger number of metal ions in the solution which results in decreasing the active surface. The active sites of the clustered adsorbent particles are not accessible by the metal ions, resulting in the decreased adsorption capacity [39, 40].

The literature survey indicate that the maximum adsorption capacity of different materials towards Cd2+ and Pb2+ are 7.28 and 7.39 [41], 80.64 and 93.45 mg/g [13], 29.23 and 31.44 [42], 92.4 and 94.5 mg/g [43] respectively, for cadmium and lead. Cadmium showed maximum adsorption capacities of 9–10 [44], 72.62 [45], 88.75 [46] 31.65 [47], 81.02 [48], 45.58 [49] and 21.6 mg/g [50]. Whereas, lead indicated maximum adsorption capacities of 97.5 [51], 417 [52], 263.6 [53] and 328.9 mg/g [54]. The literature values are summarized in Table 2.

Table 2 Literature comparison of the maximum adsorption capacities
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3.3 Kinetics studies

3.3.1 Pseudo-first order equation

The pseudo first-order and pseudo second-order kinetic models were applied to analyze the experimental data and results are summarized in Table 3. Pseudo first-order equation is shown in Eq. 2.

$$\log \left( {\frac{qe}{{qe - qt}}} \right) = \frac{{k_{1} }}{2.303}t$$
(2)

where qt and qe are the amounts of metal ions adsorbed (mg/g) at time t and equilibrium time, respectively. k1 (min−1) is the rate constant of pseudo first order kinetic equation. The values of k1 and qe are calculated from the intercepts and slopes of the linear plots of log (qe-qt) versus t. Table 3 shows that the predicted values of linear regression correlation coefficient (R2) of pseudo first-order for both Cd(II) and Pb(II) ions adsorption at all temperatures are almost ≥ 0.90. However, the difference between the theoretical and experimental adsorption capacities suggests that the kinetic data can not be interpreted by pseudo first order model. Similar results are also observed by other researchers for adsorption of metal ions from aqueous solutions using activated carbon [32, 55]. Therefore, pseudo second-order equation was applied. The pseudo second-order rate equation is shown in Eq. 3.

$$\frac{t}{{q_{t} }} = \frac{1}{{k_{2ads} q_{e}^{2} }} + \frac{t}{{q_{e} }}$$
(3)

where qt and qe are the amounts of metal ions adsorbed (mg/g) at time t and equilibrium time (mg/g), respectively. k2 (g mg−1 min−1) is the rate constant of pseudo second-order kinetic equation. The pseudo second order rate constant (k2) and the equilibrium adsorption capacity (qe) are calculated from the intercepts and slopes of the linear plots of log (t/qt) versus t. The linear plots of log (t/qt) versus t are shown in Fig. 4. The linearity of plots suggests that pseudo second order rate equation is applicable to describe the experimental data. In addition, Table 3 shows that R2 values ≥ 0.99. As such, the calculated adsorption capacity (qe) is in close agreement to the experimental values, indicating that the pseudo second order rate equation is better fitted to describe the adsorption kinetic data. Fitting of the pseudo second order rate equation also supports the chemical nature of metal ions adsorption by the USD and OSD samples [42, 43, 56].

Fig. 4
figure 4

Pseudo-second order model plots for Cd(II) and Pb(II) adsorption (pH 6; Initial Conc = 70 mg/g; Carbon Dose = 0.1 g; Shaking Speed = 110 rpm)

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Table 3 Constant values of pseudo first and pseudo second-order equations
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3.4 Mechanism of adsorption kinetics

3.4.1 Weber and Morris model

Weber and Morris model is applied to investigate the intra-particle diffusion as the rate limiting step. According to Weber and Morris [57], the intra-particle diffusion model is expressed in Eq. 4.

$$qt = K \times t^{1/2} + C$$
(4)

where qt (mg g−1) is amount of metal ions adsorbed at time t, K (mg−1 g min−1/2) is the intra-particle diffusion rate constant and C (mg g−1) is related to the thickness of the boundary layer. The value of C depicts resistance to mass transfer in the boundary layer, larger the value of C, greater is the boundary layer effect and vice versa. The values of K and C calculated from the slope and intercept of the plots of qt against t1/2, as listed in Table 3. Linear plots of qt versus t1/2 for Cd(II) and Pb(II) ions adsorption by the prepared carbon samples are shown in Fig. 5. The linearity of plots indicates that adsorption process involves intra-particle diffusion. However, intra-particle diffusion is the rate controlling step if the straight lines pass through the origin with no intercepts. When plots show multi-linearity, then the adsorption process may involve more than one step. Although the intra-particle diffusion model plots are straight lines but the straight lines do not pass through the origin. The values of the correlations coefficients R2 are also lower than 0.90. As a whole the results indicate that the intra-particle diffusion is involved but not the only rate controlling step for the adsorption of Cd(II) and Pb(II) ions onto both the activated carbon samples. The increase in the values of intra-particle diffusion rate constant (K) with increase in temperature is probably due to the increased diffusion of the metal ions into the pores of the adsorbent. It may be caused by the decrease in the viscosity of the solution with increase in temperature that leads to increased adsorption.

Fig. 5
figure 5

Weber and Morris model plots for Cd(II) and Pb(II) adsorption (pH 6; Initial Conc = 70 mg/L; Carbon dose = 0.1 g; Shaking speed = 110 rpm)

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3.4.2 Boyd model

The experimental data was not fitted in Langmuir and Freundlich isotherm models therefore, Boyd model is used to differentiate between film and intra-particle diffusion. The Boyd model is shown in Eq. 5.

$$F = 1 - \frac{6}{{\pi^{2} }}\exp \left( { - Bt} \right)$$
(5)

where F represents the fraction of solute adsorbed at any time t (F = qt/qe), and Bt is a mathematical function of F. Re-arranging the above equation, we get [Bt =  − 0.4977 − Ln (1 − F)]. The Bt values are plotted versus time t at different temperatures. When the plots are linear, passing through the origin, then the rate controlling step is pore diffusion. When the plots are non linear or linear but not passing through the origin and have intercepts, then film diffusion is the rate controlling step.

The results of Boyd model are shown in Fig. 6. It can be seen from the results that the plots of Bt versus time t are linear but not passing through the origin, which indicate that the film diffusion process is the main controlling step. Kapur and Mondal have reported similar results for the adsorption of metal ions [58].

Fig. 6
figure 6

Boyd model plots for Cd(II) and Pb(II) adsorption (pH 6; Initial Conc = 70 mg/L; Carbon dose = 0.1 g; Shaking speed = 110 rpm)

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3.4.3 Estimation of activation energy

The Arrhenius equation gives important information about the activation energy (Ea). The linearized form of Arrhenius equation can be used to calculate the activation energy of adsorption which is shown in Eq. 6.

$$\ln k_{2} = \ln A - \frac{{E_{a} }}{RT}$$
(6)

where Ea is the activation energy, R is gas constant, k2 is the rate constant obtained from pseudo second order plots, A is the pre-exponential factor and T is the solution temperature. Activation energy (Ea) values are calculated from the slopes of the plots of ln k2 versus 1/T. The values of activation energy in the range of 8.4–83.7 kJ mol−1, indicated chemical type of adsorption. Activation energy also gives concept of the rate-limiting step of the adsorption process. An adsorption process is diffusion controlled when Ea < 42 kJ mol−1 while adsorption process is considered to be chemically controlled when Ea > 42 kJ mol−1. The values of activation energy for adsorption of Cd(II) and Pb(II) ions from aqueous solutions by USD and OSD sample are listed in Table 3. Table 3 shows that the values of activation energy for Cd(II) ions adsorption by USD and OSD sample were 11.81 kJ mol−1 and 12.59 kJ mol−1 respectively while for Pb(II) adsorption were found to be 10.49 kJ mol−1 and 11.57 kJ mol−1 respectively. The calculated values of activation energy were found within the range of 8.4–83.7 kJ mol−1 indicating chemisorptions. Moreover, the values of activation energy are lower than 42 kJ mol−1, suggested that the rate limiting step is to be diffusion controlled. The metal ions from bulk solution to the adsorption sites of activated carbon surface may be transported through surface diffusion, bulk diffusion or pore diffusion. These processes are in series rather than parallel and the slowest one is the rate-controlling step which in turn determines the overall kinetics of adsorption [59].

3.5 Desorption studies

Desorption studies were performed using HCl solution which has been reported to be an efficient metal desorbing agent [60, 61]. After adsorption of Cd(II) and Pb(II) ions from their aqueous solutions of initial concentration 70 mg/L at 298 K and pH 6, each carbon sample loaded with metal ions was separated by centrifugation. The carbon was washed gently with triply distilled water to remove any un-adsorbed metal ions. The spent carbon was agitated with 70 mL of 0.1 M HCl solution for 5 h and the concentration of desorbed metal ions was determined by using atomic absorption spectrophotometer. The regeneration efficiency was investigated upto three consecutive adsorption–desorption cycles. After three cycles, the adsorption capacity of USD and OSD sample decreased from 89.83 to 67.46% and 82 to 54.6% for Cd(II) ions and 91.47% to 74.32% and 86.5 to 60.87% for Pb(II) ions respectively.

4 Conclusion

The surface oxidized (OSD) and un-oxidized activated carbon (USD) samples were derived from saw dust of the Ziziphus jujube plant by chemical activation process. The samples indicated higher BET surface area, greater pore size distribution and more surface functional groups. Based on the above properties and cost effectiveness, the OSD and USD samples were used for the successful adsorption of Cd(II) and Pb(II) metal ions from aqueous solutions. The effect of initial concentration, pH, adsorbent dose and temperature was evaluated. The uptake of metal ions increased with increase in initial pH, concentration and temperature. The adsorption capacity of un-oxidized sample was found higher than surface oxidized activated sample. As such, OSD and USD samples indicated higher uptake of Pb(II) as compared to Cd(II). In addition, both OSD and USD exhibited excellent adsorption capacities and faster adsorption rates. Pseudo second-order equation better fitted the adsorption data. Desorption study showed sufficient ease of regeneration. The overall results indicated that the prepared saw dust activated carbon samples could be used as low cost and environment friendly adsorbent for the effective removal of toxic metals ions such as Cd(II) and Pb(II) from aqueous solutions. The Ziziphus jujube activated carbon is strongly recommended for the removal of Cd(II) and Pb(II) from the industrial wastewater.

Our research group is working on the adsorption of other heavy metal ions from aqueous media using Ziziphus jujube OSD and USD samples.