The Efficiency of Cactus Leaves and Wood Charcoal as a Potential Low-Cost Adsorbent for Removal of Toxic Heavy Metals from Industrial Effluents

Presence of toxic heavy metals in the environment is of great concern due to their persistence in nature and chronic adverse effects on human health and the environment. Present paper tries to evaluate the efficiency of cactus leaves (Opuntia f. indica) and activated carbon made from acacia etbiaca as an adsorbent for the removal of heavy metal pollutants such as cadmium, lead and chromium from water. Adsorption properties such as size, dose, initial concentration and time of contact for cadmium, lead and chromium were studied through batch method. Before removing the toxic heavy metals (Cd, Pb and Cr), the fresh unpeeled cactus leaves (adsorbent) and activated carbon were washed with distilled water to eliminate the turbidity and smell from fresh unpeeled cactus. To describe the equilibrium isotherms, the experimental data were analyzed by the Langmuir and Freundlich isotherm models. Thus, the Freundlich model gave the best correlation with the experimental data. Therefore, the findings indicated that the cactus and activated carbon made locally from acacia etbiaca were found to be effective and low-cost alternative adsorbents for the removal of toxic heavy metals from industrial effluents. The preparation method allowed the use of these materials by local industries for effective remediation of pollution by removing heavy metals from their effluents.


INTRODUCTION
The toxic heavy metals are commonly present in many environments and as pollutants in many water bodies. The potential harmful effects associated with the accumulation of heavy metals in the environment are well known. The toxic effects of heavy metals on human populations and potential health hazards have been extensively studied by many workers (Jarup, 2003;Jaishankar et al., 2014). Among these, chromium(Cr), cadmium (Cd), lead (Pb) and others are frequently used in industrial processes such as tannery, textile, pharmaceuticals, metallurgy, mining, painting, smelting, batteries, and alloys industries (He et al., 2005;Tchounwou et al., 2012;Ahmed et al., 2017). Most of these heavy metals are often present at high concentration in liquid industrial waste and are often deposited directly in the environment without any pre-treatment.
The high toxicity of these metals leads to serious ecological damages, thus, there is an increasing fertilizers and waste burning. The usage of industrial effluents to agricultural activities may cause the transfer of cadmium compounds adsorbed by plants that may play a significant role in food chain, and accumulate in various human organs. Exposure to Cd affects cell proliferation, differentiation, apoptosis, human fertility and the proper functioning of DNA (Patrick, 2003;Rani et al., 2014). Cadmium causes mutations and chromosomal deletions potentially (Joseph, 2009;Pizent, 2012). Therefore, this indicated that there is a need to remove the toxicity of Cd released to the environment.
The level of such toxic contaminants of Cr, Pb and Cd in discharged wastewater should be reduced, or recycled if possible. Thus, several methods are utilized to remove toxic heavy metals from industrial waste water including chemical precipitation (Brboot et al., 2011); ion exchange (Gaikwad et al., 2010); reduction (Seaman et al., 1999); electrochemical precipitation (Tran et al., 2017); solvent extraction (Silva et al., 2005); membrane separation (Mikulasek and Cuhorka, 2016); reverse osmosis (Pires da Silva et al., 2015); biosorption (Wang and Chen, 2009;Mudhoo et al., 2012) and adsorption (Javaid et al., 2011). Among the methods commonly used for this purpose, adsorption was shown to be economically favorable and technically easy for the removal of the toxic heavy metals. Therefore, adsorption is considered as an effective and versatile method for removing toxic heavy metals like Pb, Cd, Cr, and others, particularly when combined with appropriate regeneration steps. This solves the problems of sludge disposal and renders the system more economically viable, especially if low-cost adsorbents are used (Lakherwal, 2014;Tripathi and Ranjan, 2015). Several publications utilized different inexpensive and locally abundantly available adsorbents like activated carbon (Hung et al., 2005), agricultural by-products (Sulyman et al., 2017;Mohan et al., 2005), waste materials (Kyzas, 2012) and charge minerals (Singh et al., 1992).
In order to reduce the levels of the toxic heavy metals, adsorption is rather simple, cheap and easy to extrapolate at larger scale for practical applications, which showed a good efficiency for Ethiopia. It is known cactus to be nontoxic as some of them are used in feeding animals and its fruits are used by residents of the local people, which make it good candidate for the development of processes aimed to be used in agriculture or domestic applications. Acacia etbaica is also used as energy sources in majority and medicinal purposes. Therefore, the purpose of this study were to evaluate the efficiency of cactus leaves and locally prepared wood charcoal, which are all locally available, abundant and low-cost adsorbents) in removing Cr, Cd and Pb toxic heavy metals from synthetic and contaminated water samples.

2.1.Adsorbent Preparation
Cactus (Opuntiaf. indica) was collected and the spines of the cactus leaves were removed with a knife and pads were rinsed with tap water followed by deionized water. Dissections of fresh cactus pads were performed manually using knife. The fresh Opuntia f. indica was cut into two size classes: small (1 cm) and medium (2 cm).
Dry wood logs of Acacia etbaica (A. etbaica) was cut into pieces, 50-100 cm size, and buried in earth-covered traditional kilns for weeks, where wood is cut and stacked before being covered in earth and carbonized. The charcoal was ground in a high-speed rotary cutting mill and sieved into different mm sizes: < 0.1, 0.1 -0.25, 0.25 -0.5 and 0.5 -1.0 mm. Before the application of the charcoal to our research it was washed several times with distilled water to remove dust and some other residuals. The washed samples were dried at room temperature.

2.2.Chemicals
Analytical grade chemicals were used to prepare standards solutions and were used during the analysis.

2.4.Instruments
Varian AA240 FS Fast Sequential Atomic Absorption Photometry(FED 53, USA), which is fully automated PC-controlled true double-beam Atomic Absorption spectrometer with fast sequential operation for fast multi-element flame AA determinations, features 4 lamp positions and automatic lamp selection, operated with SpectrAA Base and PRO software versions were used during the experiment.
The pH of the solutions was measured with a Hatch Lange pH meter (HANNAHI99)using a combined glass electrode calibrated with buffers of pH 2, 4, and 7. The solutions were shaken with B_HlerHorizintal Shaker (Fisher Scientific), which is thermostatic electronic shaker.

2.5.Method
Batch adsorption experiments were carried out at a laboratory room temperature for cactus and wood charcoal adsorbents at the desired initial toxic metal concentration and different pH. The required amount of the adsorbent material was then added and the flask contents were shaken at different time contacts in B_HlerHorizintal Shaker at 150 rpm (constant speed). The adsorbents filtered, centrifuged and the supernatant was analyzed for final pH and toxic metal concentration using AAS. Throughout the study, adsorption parameters of contact time, pH, dose effect, and initial concentration and adsorption isotherms were studied in batch experiment. A comparison between true wastewater samples from industrial effluents and laboratory simulated synthetic samples were also analyzed.

Adsorption Isotherm
The average amount adsorbed by the adsorbent at time t, qt (mg/g) was calculated for each experiment using the following equation: Where, Ci is the initial concentration in the solution (mg/L), Ct is the concentration in the solution at time t (mg/L, i.e., after filtration), V is the volume of solution (L), and W is the amount of adsorbent (g).
The percentage of adsorption is found from the relation: The results obtained for the adsorption of heavy metals were analyzed by the Freundlich and Langmuir models. The logarithmic form of the Freundlich model is given by the equation: Where, qe is the amount adsorbed (mg/g), Ce is the equilibrium concentration of the adsorbate (mg/L), and KF and n are Freundlich constants related to adsorption capacity and adsorption intensity, respectively (Freundlich, 1906).
Langmuir's isotherm linear equation is given by the equation (Langmuir, 1918): Where, qe represents the amount adsorbed at equilibrium time (mg/g), qmax and b are Langmuir constants that depend on the maximum monolayer adsorption capacity and adsorption equilibrium constant that relates to the energy of adsorption and Ce represents the equilibrium concentration (mg/L), respectively.
An isotherm study was performed by mixing adsorbent to the heavy metal spiked solutions at different initial concentrations while keeping all other parameters constant. The sample was withdrawn for analysis at the equilibrium time chosen based on the experiments.

3.1.Equilibrium Time
The adsorption data for the uptake of heavy metals such as Cr, Cd and Pb versus contact time

Effect of Adsorbent Particle Size
It was found that heavy metals such as Cr, Cd and Pb adsorption on fresh Opuntia. f. indica (FC) and activated carbon made from acacia etbiaca (AE) are strongly dependent of the particle size of the adsorbent. This is shown in figure 2 for a 100 mL solution containing 1 mgL -1 of Cr, Cd and Pb in contact with 6 g FC and 1g AE adsorbents and 120 min contact time. The highest percentage of removal, both when using FC and AE was observed in the treatments with the smallest particle size. The FC adsorbent sizes of 1 and 2 mm size resulted in the removal of 32% particle sizes yield a larger surface area of adsorbent, thus increasing the number of adsorption sites and enhancing the adsorption capacity (Gupta et al., 2002;Pal et al., 2006). Therefore, because of its efficient adsorption capacities and ease of separation, the size class approximately

3.3.Effect of Initial pH
The effect of pH on heavy metals such as Cr, Cd and Pb sorption by fresh cactus and activated charcoal made of acacia etbiaca was studied in the pH values of 2, 4, 7, 10 and 12 and is shown in figure 3. The adsorption studies were carried out at laboratory room temperature. A typical pH versus percentage removal of heavy metal contaminants such as Cr, Cd and Pb using FC and activated charcoal made of acacia etbiaca showed a remarkable increase in percentage removal of heavy metals at higher pH values. The same trend was also observed for Pb removal by Mbugua et al. (2016) using adsorbent materials derived from cactus leaves (O. Vulgaris cactus).
The increase in the uptake of heavy metals above pH 10.0 and thereafter no change in the adsorbed amount were observed.

Effect of Initial Heavy Metal Concentration
The effect of initial heavy metal concentration values of 0.2, 0.5, 0.75, 1.0, 1.25 and 1.5 mg/L Cr, Cd and Pb on adsorption efficiency onto FC and AE was investigated and is shown in figure   4. It is observed from the figure that the percentage heavy metal (Cr, Cd and Pb) removal decreased with the increase in initial concentrations of the corresponding heavy metal contaminants. Though the percent adsorption decreased with increase in initial heavy metal concentration, the actual amount of heavy metal adsorbed per unit mass of adsorbent increased with increase in heavy metal concentration in test solution. The initial heavy metal concentrations of Cr, Cd and Pb provides the necessary driving force to overcome the resistance to the mass transfer of the heavy metals between aqueous phase and the solid phase. The increase in initial heavy metal concentration results also an increase in the interaction between the heavy metal ions and the adsorbent (FC and AE) surface. Therefore, the increase of the initial concentration of Cr, Cd and Pb enhances the adsorption uptake of the heavy metals by the adsorbents.

Sorption Modeling
To describe the equilibrium distribution of heavy metals (Cr, Cd and Pb)  Freundlich have been used. These models were also used to calculate the loading capacity of the two forms of FC and AE adsorbents examined in this study.
Where, Ce is the equilibrium concentration of the of heavy metals (Cr, Cd and Pb) (mg/L), qe is the amount of heavy metal adsorbed per unit mass of FC and AE (g g-1), qm and KL are Langmuir constants related to adsorption capacity and rate of adsorption, respectively. The Langmuir constants can be evaluated from the slope and the intercept of linear equation (Table   1).

3.5.2.Freundlich Isotherm
This model considers a heterogeneous adsorption surface that has unequal available sites with different energies of adsorption (Freundlich, 1906) and can be represented by equation 6: Where, Kf (g/g) and n are Freundlich constants giving an indication of how favorable the adsorption process. Kf can be defined as the adsorption or distribution coefficient and represents the quantity of heavy metal (Cr, Cd and Pb) adsorbed onto the adsorbents (FC and AE) for a unit equilibrium concentration. The slope of 1/n ranging between 0 and 1 is a measure of adsorption intensity or surface heterogeneity, becoming more heterogeneous as its value gets closer to zero (Al Duri, 1995;Ayranci and Hoda, 2005 R 2 ), so the formation of more than one molecular layer of heavy metals (Cr, Cd and Pb) on the surface of fresh cactus (FC) and activated carbon made from woods of acacia etbiaca (AE) appears to be achieved.

CONCLUSION
The present study was carried out to examine the ability of fresh cactus (FC) and activated carbon made from woods of acacia etbiaca to adsorb Cr, Cd and Pb from synthetic aqueous solutions. Thus, the results of the findings indicated that the adsorbents prepared from the locally available materials appeared to be a promising adsorbent for the removal of toxic heavy metal contaminants of Cr, Cd and Pb from aqueous solutions under laboratory conditions. The optimum pH obtained for removal process was around 10. The results of this study also showed that the extent of toxic heavy metal removal (Cr, Cd and Pb) increased with decreased initial concentration of the heavy metals and also increased with increased contact time and doses of FC and AE. The adsorption process of the two studied adsorbents (FC and AE) occurred in a moderate time and reached equilibrium at about 120 min. In comparison of the two adsorbents, AE showed a higher adsorption capacity than FC under the same conditions. In addition, the results obtained were well fitted in the linear forms of Freundlich than Langmuir adsorption