Background

The removal of cadmium, Cd(II), ions is gaining wide interest from both environmental and economical viewpoints due to its serious hazardous impacts on humans, animals, and plants. There are several industries that are responsible for polluting the environment with high level of Cd(II) ions. The major sources of cadmium are products of industries such as metal plating, cadmium-nickel batteries, phosphate fertilizers, mining, pigments, stabilizers, metallurgy, ceramics, photograph, textile printing, lead mining, sewage sludge, alkaline batteries, and electroplating [1, 2]. Conventional methods for heavy metal removal from wastewater include reduction, precipitation, ion exchange, filtration, electrochemical treatment, membrane technology, and evaporation, all of which may be ineffective or extremely expensive when metals are dissolved in large volumes of solution at relatively low concentrations [3, 4]. Adsorption process is the most frequently applied method in industries for heavy metal removal. A lot of studies on this process have been carried out [5]. Many studies have recently devoted the usage of different adsorbent materials in processes involving the removal of Cd(II) ions from aqueous effluents with the aim of finding cheaper replacements for conventional sorbent material situations [6] such as activated carbon which is expensive for developing countries [7–10]. Therefore, many investigators have used inexpensive adsorbent materials, such as chitin [11], anaerobic sludge [12], apple residue [13], sawdust [14], rice polish [15], clay [16], zeolite [17], fly ash [18], chitosan [19], waste tea [20, 21], seaweeds [22], and polyaniline coated on sawdust [23].

Used papers (old newspaper, old magazine, printed papers and mixed office waste paper) are an unavoidable form of domestic waste that keeps on accumulating steadily. Existing research into the use of treated old newspaper (TNP) as adsorbent for removing heavy metal ions from aqueous solution is extremely limited by Chakravarty et.al., who studied the use of TNP as adsorbent for the removal of zinc and copper [24, 25]. This work studies the possibility of using old newspaper, a domestic waste, as an alternative low-cost sorbent in the removal of cadmium from aqueous solution and investigates the effect of initial Cd(II) concentration, contact time, dose, and pH on the adsorption process.

Methods

Preparation of treated newspaper pulp

Old newspaper was treated with concentrated sodium bicarbonate solution for removing foreign materials like grease, black ink, and bleaching material (chlorine dioxide).

The newspaper pulp (NP) was washed several times with distilled water till the pH of the supernatant water layer of the pulp was around 6.5 to 7.0. A definite amount of air-dried newspaper pulp was then refluxed with 5.0% Na2HPO4 using a water condenser for 4 h to impregnate the phosphate into the cellulosic matrix. All the parameters such as the amount of NP, concentration of disodium hydrogen phosphate, and time were optimized for maximum impregnation. After phosphorylation, the pulp was again washed with distilled water till the solution was free from phosphate. The solution was cooled and gravity filtered through a Whatman 40 filter paper. The treated newspaper pulp was air-dried and finely ground with the help of a mixer grinder to make it fluffy [24].

Adsorbate solution

All solutions used in this study were prepared from cadmium chloride by weighting and dissolving the required amount in deionized water and using 1.0 N of NaOH and HCl for pH value adjustments.

Results and discussion

Characterization of TNP

The surface functional groups and structure were studied by Fourier transform infrared spectroscopy (FTIR). The FTIR spectra of NP and TNP were recorded between 300 and 4,000 cm−1 in FTIR-8400S (Shimadzu Corporation, Nakagyo-ku, Kyoto, Japan). The FTIR spectra showed a characteristic cellulose peak in the region of 1,000 to 1,200 cm−1. The band near 1,300 cm−1 related to CH2 wagging vibrations in the cellulose. The band near 3,300 cm−1 represented the OH vibrations. The 3,700-cm−1 band in NP was seen to be split into less intense peaks in TNP due to the change in intramolecular hydrogen bonding interactions. There was a band appearing in TNP at 1,000 cm−1, corresponding to the P-O stretching (Figure 1).

Figure 1
figure 1

FTIR for NP and TNP.

Full size image

Effect of contact time

In order to establish the equilibration time for maximum uptake and to determine the kinetics of the adsorption process, cadmium adsorption on TNP was investigated as a function of contact time, and the results are shown in Figure 2. Figure 2 shows that by increasing the initial concentration of cadmium, the amount of metal uptake is also increased. The contact time was maintained for an hour to ensure that equilibrium was really achieved. It is noticed from Figure 2 that the time to reach equilibrium is almost 15 min, and it is concentration-independent. In the initial stages, the removal efficiency of the metal ion by the TNP increased rapidly due to the abundant availability of active binding sites on the sorbent, and with gradual occupancy of these sites, the sorption became less efficient in the later stages [26].

Figure 2
figure 2

Effect of contact time on adsorption uptake of Cd(II) on TNP for different initial concentrations. Temperature is at 22°C, pH 6.4, and TNP dosage is 0.33 g/L.

Full size image

Effect of TNP dosage

Figure 3 shows that the adsorption of cadmium on TNP decreased with the increasing adsorbent dosage. Cd adsorption capacity decreased gradually at the adsorbent dosage of 0.33 g with 8.41 mg/g adsorption capacity and at 1 g with 2.87 mg/g. These results may be due to the overlapping of adsorption sites as a result of adsorbent particle overcrowding [27]. Moreover, the high adsorbent dosage could impose a screening effect on the dense outer layer of the cells, thereby shielding the binding sites from metal [28].

Figure 3
figure 3

Effect of weight of adsorbent on adsorption of Cd(II) onto TNP. At pH 6.4, initial concentration of 30 mg/L, and TNP dosage from 0.33 to 1 g/L at 22°C.

Full size image

Effect of pH

Hydrogen ion concentration in the adsorption is considered as one of the most important parameters that influence the adsorption behavior of metal ions in aqueous solution. It affects the solubility of the metal ions in the solution, replaces some of the positive ions found in the active sites, and affects the degree of ionization of the adsorbate during the reaction [29]. The effect of the initial pH on the sorption of Cd(II) ions onto TNP was evaluated within the pH range of 2 to 8. Studies beyond pH 8 were not attempted because precipitation of the ions as hydroxides would be likely to occur [30]. The effect of pH on the adsorption behavior of TNP for Cd(II) is shown in Figure 4. The initial pH of the solution significantly affected the adsorption capacity of adsorbent; adsorption capacity was highest when pH was 6.4 and decreased by either the raising or lowering of pH values under the present range of experimental condition. At lower values, the metal ion uptake was limited in this acidic medium, and this can be attributed to the presence of H+ ions which compete with the Cd(II) ions for the adsorption sites. Contrarily, the metal ion was prone to Cd(OH)2 deposition through hydrolysis at higher values of pH [31].

Figure 4
figure 4

Effect of pH of the solution on adsorption of Cd(II) onto TNP. At an initial concentration of 30 mg/L and TNP dosage of 0.33 g/L at 22°C.

Full size image

Adsorption isotherms

Langmuir isotherm

Langmuir proposed a theory to describe the adsorption of gas molecules onto metal surfaces. The Langmuir adsorption isotherm has found successful applications in many other real adsorption processes of monolayer adsorption. Langmuir's model of adsorption depends on the assumption that intermolecular forces decrease rapidly with distance and consequently predicts the existence of monolayer coverage of the adsorbate at the outer surface of the adsorbent. The isotherm equation further assumes that adsorption takes place at specific homogeneous sites within the adsorbent. It is then assumed that once a Cd(II) molecule occupies a site, no further adsorption can take place at that site. Moreover, the Langmuir equation is based on the assumption of a structurally homogeneous adsorbent where all adsorption sites are identical and energetically equivalent. Theoretically, the sorbent has a finite capacity for the sorbate. Therefore, a saturation value is reached beyond which no further adsorption can take place.

The experimental data were fitted to the Langmuir equation:

C e q e = 1 qb + C e q ,
(1)

where Ce (mg/L) is the equilibrium concentration of metal ion, q e (mg/g) is the adsorption capacity in equilibrium state, q is the maximum adsorption capacity, and b is the Langmuir constant (equilibrium constant (L/mg)) which reflects quantitatively the affinity between TNP and Cd(II) ions (Figure 5). The affinity between Cd(II) and TNP adsorbent can be predicted using the Langmuir parameter b from the dimensionless separation factor RL[32]:

R L = 1 / 1 + b C 0 ,
(2)

where C0 is the initial Cd(II) concentration, and b is Langmuir isotherm constant. The adsorption process as a function of RL may be described as follows: RL > 1 unfavorable, RL = 1 linear, 0 < RL < 1 favorable, and RL = 0 irreversible. The calculated RL value for the adsorption of Cd(II) onto TNP was found to be 0.0235, which indicates a highly favorable adsorption of Cd(II) onto TNP.

Figure 5
figure 5

The Langmuir adsorption isotherm for Cd(II) adsorption onto TNP.

Full size image

Freundlich isotherm

The Freundlich adsorption equation has the following general form:

ln q e = ln K F + 1 / n ln C e ,
(3)

where K F and n are the isotherm parameters to be determined. The Freundlich adsorption isotherm represents the relationship between the corresponding adsorption capacity q e (mg/g) and the concentration of the metal in the solution at equilibrium C e (mg/L) (Figure 6).

Figure 6
figure 6

The Freundlich isotherm for Cd(II) adsorption onto TNP.

Full size image

From Table 1, it is shown that both models of Langmuir isotherm and Freundlich isotherm have a narrow value of R2 which varies from 0.8906 for Langmuir to 0.8697 for Freundlich. Thus, it is concluded that the Langmuir model is an appropriate model to represent the adsorption equilibrium data. The Freundlich constant n is found to be higher than 1, which indicates that the adsorption of Cd(II) on TNP is favorable.

Table 1 Fitted parameters of Cd(II) onto TNP adsorption system
Full size table

Adsorption kinetics

Kinetics of adsorption is one of the most important characteristics that is responsible for the efficiency of adsorption. The adsorbate can be transferred from the solution phase to the surface of the adsorbent in several steps, and one or any combination of which can be the rate-controlling mechanism: (1) mass transfer across the external boundary layer film of liquid surrounding the outside of the particle, (2) diffusion of the adsorbate molecules to an adsorption site either by a pore diffusion process through the liquid-filled pores or by a solid surface diffusion mechanism, and (3) adsorption (physical or chemical) at a site on the surface (internal or external), and this step is often assumed to be extremely rapid.

The overall adsorption can occur through one or more steps. In order to investigate the mechanism of process and potential rate-controlling steps, the experimental kinetic data for the uptake of naphthalene at different initial concentrations, which is modeled by the pseudo-first order by Lagergren and the pseudo-second order by Ho and McKay [33], are given in Equations 4 and 5, respectively:

log q e − q t = log q e − K 1 2.303 t ,
(4)
1 q t = 1 k 2 q e 2 1 t + 1 q 6 .
(5)

The pseudo-first-order model was used to check the adsorption data of Cd(II) on TNP, but the correlation coefficient was not high. However, the pseudo-second-order kinetic model [33] was successfully applied with a very high correlation coefficient for explaining the kinetic data of the adsorption processes [34]. The adsorption of Cd(II) on TNP could be a pseudo-second order process rather than a first order.

By comparing the figures of the pseudo-first order (Figure 7) and the pseudo-second order (Figure 8) models, it is obvious that the pseudo-second-order model fits the experimental data better than the pseudo-first order for the entire adsorption period. Also, from Table 2, the values of q e obtained from the pseudo-second-order model are closer to the experimental results than q e obtained from the pseudo-first-order model. By comparing the coefficient of determination R2 in Tables 2 and 3, it is observed that the pseudo-second-order model fits the experimental data with higher R2 values (0.9911 to 0.9975) than the pseudo-first order R2 values (0.7299 to 0.7203). The higher R2 values confirm that adsorption is well represented by the pseudo-second-order model.

Figure 7
figure 7

Fitting of pseudo-second-order model for Cd(II) adsorption onto TNP at 22°C, 0.33-g dosage, and pH 6.4.

Full size image
Figure 8
figure 8

Fitting of Lagergren's pseudo-first-order model for Cd(II) adsorption on TNP at 22°C, 0.33-g dosage, and pH 6.4.

Full size image
Table 2 Pseudo-second order at different initial Cd(II) concentrations
Full size table
Table 3 Pseudo-first order at different initial Cd(II) concentrations
Full size table

Experimental

Adsorption kinetic experiments

Kinetic adsorption experiments were carried out to investigate the effect of time on the adsorption process and to identify the adsorption parameters. The experimental procedures were as follows:

  1. 1.

    A volume of 750 mL of cadmium chloride solution of various concentrations (5, 10, 15, and 30 mg/L) were prepared.

  2. 2.

    The cadmium chloride solution was placed in thermostatically controlled 2-L beaker, and 0.33 to 1 g of TNP was added to the solution and stirred by a rectangular impeller at a constant speed of 300 rpm for a definite shaking time varied between 0 and 30 min.

  3. 3.

    After shaking, the solution was allowed to settle for 30 min, filtered, and analyzed for cadmium by atomic absorption spectroscopy.

  4. 4.

    The difference in the adsorbate (cadmium) content before and after adsorption represented the amount of adsorbate adsorbed by TNP.

The adsorption capacity (q) was calculated using the following formula:

q = C 0 − C W × V ,
(6)

where C0 is the initial concentration of metal ion in the solution (mg/L), C is the concentration of metal ion in solution (mg/L), V is the total volume of solution (L), and W is the TNP dosage (g).

Conclusions

The adsorption behavior of cadmium onto the TNP from aqueous solution has been investigated in this study. The present study shows that TNP can be used as low-cost adsorbent for the removal of cadmium from aqueous solution. The results suggest that The amount of cadmium removed changes with initial Cd(II) concentration and contact time. Maximum adsorption occurs at the Cd(II) concentration of 30 mg/L. The adsorption process increases with the increasing the pH value until it reaches 6.4 and then it decreases. The adsorption process increases with the increasing dose of TNP. The Langmuir and Freundlich isotherms can be used to describe the adsorption equilibria of cadmium onto TNP. It was found that the equilibrium adsorption data were more fitted to the Langmuir isotherm than to the Freundlich model. The kinetics of the adsorption were found to be fitted with pseudo-second-order model.

Authors' information

MO got her B.Sc. and M.Sc. in Chemical Engineering from Alexandria University, Alexandria, Egypt. She received her Ph.D. from Wayne State University, Detroit, MI, USA. At present, she is an associate professor at the Petrochemical Engineering Department in Pharos University. MM got his B.Sc., M.Sc., and Ph.D. in Chemical Engineering from Alexandria University, Alexandria, Egypt. He is currently working as an assistant professor in the Chemical Engineering Department at Alexandria University.