Removal of metallic elements from industrial waste water through biomass and clay

Abstract

This study reports the removal of nickel(II) and copper(II) ions (Ni²⁺ and Cu²⁺) from aqueous solution using pure and chemically pretreated biomass from Arachis hypogea (peanut shells), Prunus amygdalus (almond shells), Arundo donax (giant cane) and two clay materials, clay G and clay B. These materials are indigenous, easily available, surpulus by-products for biosorption studies. Batch experiments were carried out to determine the effect of various adsorbent factors such as initial pH, temperature, particle size and contact time on the adsorption process. For adsorption application, up to 99% removal of both metal ions was achieved by biomass and clay materials. Furthermore, chemically modified adsorbents significantly increased the uptake capacity of biomass, suggesting that the affinity between metal and sorbent can be increased after pretreatment. Equilibrium isotherms were analyzed using Langmuir and Freundlich isotherm models, and both models fitted to explain the adsorption behavior of metal ions on to biomass and clay. This shows that the adsorption of metal ions on the adsorbent is a physical adsorption mechanism. In conclusion, owing to its outstanding nickel(II) and copper(II) uptake capacity, the utilized biomass proved to be an excellent biosorbent.

Keywords:

• ions
• biomass
• clay
• peanut shells
• almond shells
• modified
• biosorption

Introduction

Pollution, defined as an undesirable change in the natural functioning of the environment owing to the addition of detrimental particulates, can be categorized into three types: environmental (land), atmospheric (air) and marine (water). Water covers 70% of the earth's surface so in great danger of contamination, which creates major problems. Waste water is a combined term used for spent water released from the community, homes, farms and factories. It usually contains organic compounds, heavy metals, sewage organisms, including bacteria and fungi, and petroleum products, along with dissolved and suspended particles, nutrients and hazardous materials. One of the major sources of waste water pollution is industrial waste water. Many industries, such as textile, pulp and paper, tanning, sugar, fertilizer and rubber industries, release huge volumes of waste water (Haq et al. 2011). The use of large volumes of waste water by millions of large-scale farms has increased owing to a lack of any alternative source for irrigation purposes.

‘Heavy metals’ is a general collective term applied to the group of metals and metalloids with an atomic density greater than 6 g cm⁻³, which is widely recognized and usually applied to elements such as cadmium, chromium, copper (Cu), mercury, nickel (Ni), lead and zinc, that are commonly associated with pollution and toxicity problems (Connell et al. 2008). These toxic chemicals in the waste water have adverse effects on human health and the environment. Extensive use of waste water for irrigation creates an issue of concern to agencies responsible for maintaining public and environmental health (Afkhami et al. 2008).

Plants play an important role in cleaning up soil, sediment and water polluted with heavy metals. However, this is a long-term procedure and regeneration of plants, or phytoremediation, is difficult. Plants cannot be used as a cost-effective treatment method for heavy metal removal as they achieve incomplete metal removal (Ahalya et al. 2003). Nickel (II) and copper (II) are known environmental pollutants, and their removal from waste water is of great importance. Conventional methods such as precipitation, oxidation/reduction, ion exchange, filtration, membranes and evaporation are extremely expensive or inefficient for metal removal from dilute solutions containing 1–100 mg l^–1 of dissolved metal.

The search for new techniques is focusing on biosorption, which is the removal of metal or metalloid species, compounds and particulates from solution by biological material. Adsorption is evolving as a front line of defense. Selective adsorption utilizing biological materials, mineral oxides, activated carbons or polymer resins has generated increasing excitement (Gadd 1990). Adsorption is a well-established separation technique, defined as the ability of the adsorbate to attach with the adsorbent. The technique works not only for the removal of dilute pollutants but also for the recovery of useful products from aqueous streams (Chia & Shen 2000). Adsorption is divided into two types. One is physiosorption or physical adsorption, and other is chemisorption, which is much stronger than physiosorption as molecules are held to the surface by valence forces. Heat evolved during the chemosorption process ranges from 10 to 100 kcal mol⁻¹, whereas physiosorption evolves less than 5 kcal mol^–1 (Motoyuki 1990).

Biosorbents show different affinities towards sorbate species (sorbate, e.g. metal ions). The biosorption process involves a solid phase (sorbent or biosorbent, i.e. biological material) and a liquid phase (solvent, normally water). Owing to the higher affinity of the sorbent for the sorbate species, the latter is attracted and bound into the solid (Kratochvil & Velosky 1998). The rate of biosorption depends on the type of process. Various mechanisms can operate simultaneously to varying degrees in biosorption (Gadd 2009). The origin of the biomass should be kept in mind when designing a metal biosorption experiment (Volesky 1987). Considering the number of types of biomass and the metal of interest, all multiplied by the number of experimental parameters, there is very large scope for potential new metal biosorbents (Volesky & Holan 1995). Cassia fistula, used as a biosorbent for Ni²⁺ uptake from aqueous solution, showed excellent removal capacity (Hanif et al. 2007; Mushtaq et al. 2014). For cadmium, lead and zinc removal, Aspergillus flavus biomass proved excellent in a shaking flask experiment (Kok et al. 2001; Tahira et al. 2015). Photosynthetic bacteria such as Rhodobacter sphaeroides S and a marine photosynthetic bacterium, Rhodovulum sp. PS88, in a batch culture system achieved a high removal ratio of cadmium from culture medium under anaerobic and aerobic conditions (Watanabe et al. 2003). The present research visualizes the use of dead biomass for the removal of heavy metals from aqueous solutions. To account for the problem of heavy metals, different materials have been applied as cheap biosorbents.

High levels of water pollution can destroy aquatic life. Pollutants are taken up by plants and animals and can enter the human body, resulting in many health problems. The situation is worse in countries where people do not have access to potable water, and in many instances polluted water is used as a source of drinking water (Faryal et al. 2015; Kousar et al. 2015).

In this article, several different adsorbents were tested: shells of almond (Prunus amygdalus), an important stone fruit grown in Pakistan; shells of peanut or groundnut (Arachis hypogaea), a species in the legume or ‘bean' family (Fabaceae), which is an annual herbaceous plant growing 30–50 cm (0.98–1.6 feet) tall; giant cane (Arundo donax), a tall perennial cane that grows in damp, either fresh or moderately saline, soils (Bell 1997; Alden et al. 1998); and clay material (clay G and clay B) doped with different silicate minerals of heavy metals, which has been shown to be excellent in the removal of heavy metals from aqueous effluents. These adsorbents are inexpensive and easily available throughout the year, and offer a cost-effective alternative to the conventional treatment of waste water from industry (Ashraf et al 2013a). The main objective of this study was to study the efficiency of biosorbents in the removal of heavy metals and to assess their potential as a cheap treatment method.

Material and methods

Reagents

For both metals (Ni²⁺ and Cu²⁺), stock solutions of their chlorides (1000 mg l^–1) were prepared by dissolving molar amounts in volumetric flasks. From the stock solutions, intermediate solutions of different concentration were prepared in double-distilled water by applying the dilution formula: (1) $C_1{V}_1=C_2{V}_2$(1)

Acidic solutions of 0.1 M HCl, 0.1 M CH₃COOH, 0.1 M methanol and 0.1 M acetone were prepared using the following formula: (2) $\mathrm{Molarity}\mathrm{of}\mathrm{acid}=\frac{(\mathrm{\%}\mathrm{Purity}\left)\right(\mathrm{Specific}\mathrm{gravity})\times 10}{\mathrm{Molecular}\mathrm{weight}}$(2)

Concentrated acid was diluted using M₁V₁ = M₂V₂. All the chemicals used were of analytical grade from the Merck Index.

Pretreatment of biomass

All the adsorbents of plant origin used in the study were obtained from a local market and prepared before being used for treatment. Arachis hypogaea (peanut) shells were washed thoroughly to remove dust and foreign particles, then washed with distilled water, dried in sunlight for 7 days, and ground to a fine powder with a grinder. Washed P. amygdalus (almond) shells were dried in sunlight for 15 days then oven-dried at 150°C. Shells were first ground in a pestle and mortar, then ground to a powder in a grinder. Arundo donax (giant cane) was washed and dried in sunlight for 10 days, then in an oven at 50°C, and finally ground to a powder in a grinder.

Modification of clay

Clay material (clay G and B) doped with different silicate minerals was modified by treatment with nitric acid and kept for 6 hours in sunlight. Once dry, it was cooked at 1500°C for 24 hours continuously. Cooked clay was modified into a granular mass.

Batch biosorption studies

Batch experiments for nickel(II) and copper(II)

Batch experiments were carried out using a series of Erlenmeyer flasks of 100 ml and 250 ml capacity. The effects of pH (2–6), contact time (1.30 min), initial nickel and copper concentrations (2 ppm, 4 ppm and 6 ppm), adsorbent dose (0.05 g, 0.1 g, 0.2 g and 0.3 g) and temperature on adsorption of the material ions from the solutions were investigated during the study. All the adsorption experiments were carried out at room temperature (30 ± 1°C), except where the effect of temperature being investigated. All the flasks were placed on a rotating shaker (PA 250/25 H) with a constant shaking speed of 120 rpm. At the end of the experiment, the flasks were removed from the shaker and the solutions were separated from the biomass by filtration through filter paper (Whatman no. 40, ashless) followed by centrifugation. To adjust the pH of the medium, 0.1 N solutions of NaOH and HCl were used.

Cellulose modification

Adsorbent (almond shells, 100 mesh size) was washed in acetone and dried in an oven at 90°C for 1 hour. Then, using a 100 ml reflux set, 2.2 g of adsorbent, 8.2 g of succinic anhydride and 42 ml of pyridine were added. A condenser was attached and reflux was run for 24 hours. After completion of the reaction, Gooch filtration was performed, followed by washing. The washed modified biomass was dried in an oven at 80°C for 1 hour and left in a desiccator overnight.

Cellulose (peanut shells) preparation chemically modified with succinic anhydride

Adsorbent (peanut shells, 20 mesh size) was washed with acetone and dried in an oven at 80°C for 1 hour. Using a reflux set, 5 g of biomass, 16 g of succinic anhydride and 84 ml of pyridine were added. Reflux was run for 24 hours. The prepared biomass was filtered using Gooch filtration apparatus. The modified product was dried in an oven at 89°C for 1 hour and placed in a desiccator overnight.

Modification in NaOH/urea

Adsorbent was washed with acetone and dried in an oven at 90°C for 40 minutes. A mixture of 6 g NaOH, 4 g urea in 90 ml distilled water and 5 g of adsorbent (almond shells, 100 mesh size) was refluxed for 48 hours. The solution was then filtered and washed with water. The same procedure was repeated for 10 mesh size peanut shells.

Uptake capacity

The quality of sorbent material is judged according to how much sorbate it can attract and retain in an immobilized form. For this, it is customary to determine the metal uptake sorption capacity (q) by the adsorbent as the amount of sorbate bound by the unit of solid phase. The amount of metal bound by the sorbent which disappears from the solution is calculated based on the mass balance for the sorbent in the system, using Equation (3(3) $q=\frac{V(C_i-C_f)}{S}$(3) ): (3) $q=\frac{V(C_i-C_f)}{S}$(3) where q is metal ion uptake capacity (mg g⁻¹), C_i is the initial concentration of metal in solution before the sorption analysis (mg l⁻¹), C_f is the final concentration of metal in solution after the sorption analysis (mg l⁻¹), S is the dry weight of adsorbent (g), and V is the solution volume (l).

All the samples were analyzed for nickel and copper concentrations using a Perkin Elmer A Analyst atomic absorption spectrophotometer.

Statistical analysis

Langmuir and Freundlich models were used to characterize the adsorption for single metal component systems. The Langmuir model makes assumptions such as monolayer adsorption and constant adsorption energy, while the Freundlich model deals with heterogeneous adsorption. The Langmuir equation of adsorption is shown in Equation (4(4) $q=\frac{q_{\max }b{C}_f}{(1+b{C}_{f)}}$(4) ): (4) $q=\frac{q_{\max }b{C}_f}{(1+b{C}_{f)}}$(4)

It was linearized to the form shown in Equation (5(5) $\frac{1}{q}=\frac{1}{q_{\max }}+\frac{1}{(b.q_{\max })\left(C_f\right)}$(5) ): (5) $\frac{1}{q}=\frac{1}{q_{\max }}+\frac{1}{(b.q_{\max })\left(C_f\right)}$(5) where q_max and b are the Langmuir constants.

The Freundlich equation of the adsorption isotherm is shown in Equation (6(6) $q=K(C_f{)}^{1/n}$(6) ): (6) $q=K(C_f{)}^{1/n}$(6)

Its linearized form is represented by Equation (7(7) $\log q=\log K+\left(\frac{1}{n}\right)\log C_f$(7) ): (7) $\log q=\log K+\left(\frac{1}{n}\right)\log C_f$(7) where q is the amount adsorbed per unit mass of adsorbent and C_f is the equilibrium concentration. The plot of log q versus log C_f is linear, and constants K and n are evaluated from slopes and intercepts.

Results and discussion

This study tested the efficiency of five non-conventional adsorbents of plant origin and clay material for the removal of metal ions, nickel (Ni²⁺) and copper (Cu²⁺), from industrial waste water. The main purpose was to develop cheap and useful metal ion adsorbents that are readily available and economically affordable. Five adsorbents – peanut shells (A. hypogea), almond shells (P. amygdalus), giant cane (A. donax) and two clay materials – were tested for their metal ion removal efficiency in aqueous solution. The results for biosorption of Ni²⁺ by A. hypogea as a function of initial concentration of control flasks or blank solutions were expressed as Langmuir and Freundlich isotherms. Uptake capacity increased to 0.6314 mg g^–1 and removal of Ni²⁺ from solution was higher at low initial concentration, with a maximum uptake of 98.70% at 1 mg l⁻¹ concentration. Both models describe the experimental data as the value of the coefficient of determination, being 0.9619 and 0.972 for the Langmuir and Freundlich isotherms, respectively. The graph reveals the relationship between initial concentration, percentage removal and metal ion uptake capacity.

The rate of adsorption is a function of the initial concentration of metal ions, which makes it an important factor to consider for effective adsorption (Ahalya et al. 2005; Ashraf et al. 2010; Ashraf et al. 2011). From Table 1, in general, the data reveal that the sorption capacity increased with increasing initial metal ion concentration. The table also shows a comparison of uptake capacity (mg g⁻¹) and percentage removal of metal ions by different adsorbents on sorbents. This sorption characteristic shows that surface saturation was dependent on the initial metal ion concentrations. At low concentrations, adsorption sites took up the available metal more quickly. However, at higher concentrations, metal ions need to diffuse to the biomass surface by intraparticle diffusion and greatly hydrolyzed ions will diffuse at a slower rate (Horsefall & Spiff 2005). It was found that when the metal ion concentration was low the metal ion adsorption rate increased, whereas when the metal ion concentration was high the metal removal rate decreased. Tables 2 and 3 show individual uptake capacity values for Cu²⁺ and Ni²⁺ for all adsorbents. The order of selectivity for metal ions towards the studied adsorbents observed was Ni > Cu.

Table 1. Comparison of Langmuir and Freundlich isotherm models for Ni²⁺ and Cu².

		Langmuir isotherm parameters	Experimental value	Freundlich isotherm parameters
Adsorbent	Metal	qmax	R^2	qmax	K	1/n	R^2
Prunus amygdalus	Ni^2+	1000	0.957	0.7442	2.989	0.334	0.960
	Cu^2+	1000	0.970	0.6941	2.865	0.460	0.967
Arachis hypogea	Ni^2+	1000	0.991	0.7492	2.892	0.368	0.955
	Cu^2+	1000	0.972	0.7092	2.918	0.389	0.961
Arundo donax	Ni^2+	1000	0.998	0.7478	2.937	0.394	0.961
	Cu^2+	1000	0.977	0.6600	2.763	0.307	0.955
Clay G	Ni^2+	1000	0.959	0.7608	2.744	0.366	0.987
	Cu^2+	1000	0.952	0.6914	2.874	0.429	0.963
Clay B	Ni^2+	1000	0.961	0.6248	2.732	0.358	0.960
	Cu^2+	1000	0.960	0.56934	2.073	0.999	0.977

Table 2. Adsorption of Cu²⁺ on chemically modified material.

Components	Cf (mg l^−1)	q (mg g^−1)	%
Unmodified	0.1553	0.124	94.82
Modified	0.754	0.110	97.48
Modified (G)	0.894	0.291	97.02

Note: C_f = final concentration of metal in solution; q = metal ion uptake capacity; G = gel.

Table 3. Adsorption of Ni²⁺ on chemically modified material.

Component	Cf (mg l^−1)	q (mg g^−1)	%
Modified	1.089	0.230	78.22
Modified (G)	1.152	0.274	79.24

Note: C_f = final concentration of metal in solution; q = metal ion uptake capacity; G = gel.

Figures 1 and 2 show the effect of initial pH on the adsorption of metal ions to the adsorbent. Metal ion adsorption by most adsorbents used in the study did not occur below pH 2. Metal adsorption between 2 and 5 was optimal. From Figure 1, the adsorption pH for Ni²⁺ was 4 for most adsorbents. Above pH 7, precipitation of Ni²⁺ was evident (Ashraf et al. 2012; Ashraf et al. 2013b). Similar behavior was shown by the leached biomass but the percentage of Ni²⁺ removal was much higher than by the fresh biomass. The increase in percentage removal of metal ions due to increased pH may be explained on the basis of a decrease in competition between protons (H⁺) and positively charged metal ions at the surface sites, and also by a decrease in positive charge near the surface, which results in a lower repulsion of the adsorbing metal ion. Maximum percentage removal of nickel was observed at pH 4, which could be due to the formation of M(OH) and M(OH)₂ as hydrolyzed products. The lower solubility of these hydrolyzed species may be another reason for maximum adsorption. The results are supported by a previous study conducted on Cassia fistula biomass in its natural form, whose uptake capacity for Ni²⁺ increased at pH 3–6 and decreased at pH 7–8, showing 99–100% removal efficiency (Hanif et al. 2007). The results also show that the uptake of free ionic Ni²⁺ depends on pH. It was observed that the uptake of Ni²⁺ increased with the increase in pH by 99% over the pH range 2–5. This increase may be due to the presence of negative charge on the surface of the adsorbent, which may be responsible for metal binding. However, as the pH falls, the hydrogen ions compete with the metal ions for the sorption sites in the sorbent; the overall surface charge on the particles becomes positive and hinders the binding of positively charged metal ions. On the other hand, the decrease in adsorption under pH 4 may be due to occupation of the adsorption sites by anionic species, which impedes the approach of such ions towards the sorbent surface (Ashraf et al. 2013b; Anjum et al. 2015). Copper showed different behavior at different pH values for all adsorbents. Copper adsorption increased as the pH increased.

Figure 1. Comparison of all adsorbents at different pH for Ni²⁺.

Figure 2. Comparison of all adsorbents at different pH for Cu²⁺.

The results for acid–base-modified peanut shells as adsorbents for metal removal indicate 19–34% higher average uptake values than unmodified shells. Peanut shells modified by 0.6 M citric acid or 0.6 M phosphoric acid have shown promise as metal ion adsorbents. The adsorption of Cu²⁺ and Ni²⁺ ions from aqueous solutions on to activated carbon from peanut shells was also studied as a function of the concentration of the ions and the pH value (Luis et al. 2009).

Chemical modification of peanut and almond shells

In the first experiment, adsorption of heavy metals from aqueous solution using adsorbents (almond shells) chemically modified with succinic anhydride resulted in the formation of different components, separated by filtration. Separated components were residue, modified product and gel material. Different tests were performed on these components.

Melting point

The melting point for the residue showed that instead of melting, it decomposed at 199°C. The gel became red hot at 180°C and the modified product decomposed at 280°C.

Solubility

Components were tested for their solubility in different solvents, e.g. water, methanol, ethanol, acetone, dimethylglyoxime (DMG), dimethyl sulfoxide (DMSO), carbon tetrachloride (CCl₄) and chloroform (CHCl₃). The residue was insoluble in all solvents, but soluble in acetone and DMSO on heating. The product was also insoluble in all solvents, except for DMG and DMSO.

Thin-layer chromatography

All three components were tested by thin-layer chromatography. Some differences in the flow rate were seen, suggesting changes in the structure of the adsorbent. Functional groups may show some changes and new sites may be present.

Adsorption

Experiments were also performed for adsorption of Ni²⁺ and Cu²⁺ from aqueous solution using material chemically modified with succinic anhydride (almond shells). The modified product was analyzed and the results are shown in Table 2 and 3. These tables show the concentration of copper and nickel adsorbed on to the product. The initial concentration of copper was 3 ppm and that of nickel was 5 ppm. Modified product and gel adsorbed about 97% of the metal. Both modified products also show adsorption to nickel of up to 80%. The results were better for copper metal than for nickel. These results suggest that considerable changes have occurred in the modified product.

The same experiment was performed on peanut shells, and the results are shown in Table 4. The table shows the concentration of copper and nickel adsorbed on to the peanut product. The initial concentration of copper was 3 ppm and that of nickel was 5 ppm. Unmodified reagent adsorbed 66.31% of Cu²⁺, modified product adsorbed 81.90% of Cu²⁺, and other washed materials adsorbed a maximum of 62.53% and 91% of Cu²⁺.

Table 4. Adsorption of Cu²⁺ on chemically modified material from peanut shells.

Adsorbent	Cf (mg l^−1)	q (mg g^−1)	%
Ethanol washed	0.6228	0.237	79.24
Mix-washed peanut shell (P)	1.124	0.187	62.53
Water-washed peanut shell (P) absorbed	0.2708	0.272	90.97333
Modified (P)	0.5428	0.245	81.90667
Unmodified (R)	1.0107	0.266	66.31

Note: C_f = final concentration of metal in solution; q = metal ion uptake capacity; P = modified product; R = residue.

Both modified products also showed adsorption to Ni²⁺. The results are shown in Table 5. Modified product showed maximum adsorption of 98% of Ni²⁺. The results were better for Ni²⁺ than for Cu²⁺.

Table 5. Adsorption of Ni²⁺ on chemically modified material from peanut shells.

Adsorbent	Cf (mg l^−1)	q (mg g^−1)	%
Reactant (R) absorbed	1.710	0.383	65.80
Modified adsorbed	0.0969	0.490	98.06
Mix-washed peanut shell	.0660	0.434	86.80
(P) absorbed
Ethanol washed	1.0069	0.399	79.86

Note: C_f = final concentration of metal in solution; q = metal ion uptake capacity; P = modified product; R = residue.

Another experiment was performed on biomass in which the product was chemically modified with NaOH/urea. The results are shown in Table 6. Table 6 shows the adsorption of products formed during experiments on Cu²⁺. Two adsorbents (peanut shells, almond shells) were tested in the experiment. Peanut shells showed better results than almond shells.

From the above three experiments, it is concluded that there is the possibility of some modification in the structure of adsorbents.

Table 6. Synthesis of adsorbents in NaOH/urea aqueous solution.

Adsorbent	Cf (mg l^−1)	q (mg g^−1)	%
Almond shell (P)	0.1752	0.160	94.16
Peanut shell (P)	0.1514	0.232	94.95
Ethanol-washed peanut shell (P)	0.5575	0.244	81.41
Water-washed almond shell (P)	0.7957	0.220	73.47
Acetone-washed peanut shell	0.8264	0.217	72.45

Note: C_f = final concentration of metal in solution; q = metal ion uptake capacity; P = modified product.

Comparison of unmodified and modified adsorbents

Removal percentages of metals in the first two experiments were 94.82% and 66.31% Cu²⁺ and 65.8% Ni²⁺ on unmodified adsorbent, and 97.48% and 81.90% Cu²⁺ and 78.22% and 98.06% Ni²⁺ on modified adsorbent. In the third experiment, almond shells and peanut shells showed 94.16% and 94.95% Cu²⁺ removal.

Applications of adsorbents

Five adsorbents were applied on industrial effluent from electroplating and tanning to remove heavy metals such as Cu²⁺ and Ni²⁺.

Electroplating effluent contained initial concentrations of 1.5247 ppm Cu²⁺ and 1.7294 ppm Ni²⁺. The adsorbents used in this study, P. amygdalus, A. hypogea, A. donax, clay G and clay B, showed metal uptake concentrations of 0.0057 ppm, 0.0093 ppm, 0.0072 ppm, 0.0077 ppm and 0.0275 ppm Cu²⁺, and 0.0031 ppm, 0.0105 ppm, 0.0142 ppm, 0.01875 ppm and 0.0252 ppm Ni²⁺, respectively.

Tannery waste contained initial concentrations of 1.6324 ppm Cu²⁺ and 1.2460 ppm Ni²⁺. The adsorbents, P. amygdalus, A. hypogea, A. donax, clay G and clay B, showed metal uptake concentrations of 0.8210 ppm, 0.9722 ppm, 0.8124 ppm and 0.5754 ppm Cu²⁺, and 0.7651 ppm, 0.0523 ppm, 1.0372 ppm, 1.1982 ppm and 1.1324 ppm Ni²⁺, respectively. These results are in agreement with the study by Ilhan et al. (2004).

Conclusion

All of the adsorbents used in the study proved useful for the removal of metal ions from industrial waste water. Of the adsorbents tested, P. amygdalus showed better results as a biosorbent for Ni²⁺ ions than for Cu²⁺, after some chemical modification. Further studies on these adsorbents are recommended.

Disclosure statement

The authors certify that there is no conflict of interest with any financial organization regarding the material discussed in the paper.

Additional information

Funding

This research is supported by a University of Malaya research grant, UMRG [RG257-13AFR]. Thanks also for the support by IPPP [PG133-2014B and PG138-2014B].