1 Introduction

Along with the rapid development of mining, metallurgy, and other industries, the production and discharge of heavy metal wastewater and derived pollution have become a global environmental problem. The composition of heavy metal wastewater is complicated, and its toxicity is severe. Heavy metals can interact strongly with proteins and enzymes in the human body through the food chain and accumulate in human organs. Once the concentration of heavy metal ions in the body exceeds the human body’s tolerance limit, it causes toxic reactions in the human body. The treatment of heavy metal pollution becomes imminent. In recent, removing heavy metal ions from wastewater has been a concern for health and environmental problems. For the low concentration of heavy metal ions, the traditional chemical precipitation method and ion exchange resin method are limited in the purification effect. Instead, it was disclosed that the adsorption method could effectively and solve this problem with improved purification. Adsorption method has become more interested in removing heavy metal ions due to its advantages of low cost, high efficiency, environmental protection, and recyclability of heavy metals [1, 2]. Some research reported the modification of absorbent by using physical and chemical methods and showed the adsorbent’s enhanced selectivity to heavy metal ions [3,4,5].

Coconut shell is easily obtained from agriculture and forestry and has low cost, dense structure, low content of ash content, and high strength. It has been used to remove heavy metal ions from water [6,7,8,9], organic matter [10], and separate and purify mixed exhaust gases of CO2 and CH4 [11,12,13]. In this study, four different reagents of nitric acid, sodium hydroxide, potassium permanganate, and iron salt were used to modify coconut shell-based activated carbon to investigate the purification effect of heavy metals from wastewater. The reaction time, adsorbent input amount, and pH value on the purification were experimentally investigated. Figure 1 depicts the processes in this research from materials preparation, modification, and absorption reaction.

Fig. 1
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The schematic diagram for the improved removal effect of Cu2+, Cd2+, Pb2+, and Zn2+ ions on wastewater

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Fig. 2
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SEM images of surface morphology on coconut shell-based activated carbons; ordered in AC-0 through AC-5

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Fig. 3
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Effect of pH on adsorption performance of different types of coconut shell-based activated carbon; ordered in AC-0 through AC-5

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Fig. 4
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Effect of adsorbent dosage on adsorption performance of different types of coconut shell activated carbon; ordered in AC-0 through AC-5

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2 Materials and methods

2.1 Material preparation of coconut shell-based activated carbon

The coconut shell-based activated carbon used in experiments was purchased from gongyi guo qing shui material co., LTD. The physical and chemical properties are shown in Table 1. About 500 g of coconut shell-based activated carbon was mixed with ionized water in a 1000-mL beaker, heated to boil for 30 min with a gentle stir, and cooled down. After taking it out, it was rinsed with deionized water 4–5 times and then filtered. The cleaned activated carbon was placed at 383 K and dried for 12 h to obtain an activated carbon sample (named AC-0).

2.2 Modification of activated carbon by different agents

The prepared activated carbon of coconut shell (AC-0) was immersed in HNO3 and 4% NaOH solution with a mass concentration of 1% and oscillated 24 h. After washing, filtration, drying, nitric acid, and sodium-hydroxide modified activated carbon were prepared (named as AC-1 and AC-2, respectively).

The prepared activated carbon of coconut shell (AC-0) was immersed in 1.5% KMnO4 solution. After 12-h oscillation under room temperature, the modified coconut shell activated carbon was washed with ultra-pure water and then dried in a 110 °C constant temperature oven for 12 h to prepare potassium permanganate modified activated carbon (named as AC-3).

About 2 g of AC-0 was oscillated for 24 h in FeSO4 solution with a concentration of 50 mL of 0.04 mol/L. After filtration, it was washed with deionized water until no color and dried in a 40 °C oven for 4 h. The iron salt modified activated carbon was prepared (named as AC-4).

2.3 Characterization of surface morphology of activated carbon

The surface morphology of coconut shell-based activated carbon before and after modification (AC-0 through AC-4) was characterized using scanning electron microscopy—Zeiss GeminiSEM 300.

3 Adsorption experiments

Each coconut shell-activated carbon samples were weighed and placed in a 250-mL conical flask, and a 50-mL beaker containing 5 mg/L of Cd2+, Cu2+, Pb2+, and Zn2+ solution was added. The beaker was put on a constant temperature oscillator with 25 ± 1 °C. After adjusting the pH value, it then oscillated at a speed of 200 rpm. The supernatant solution was filtered with a 0.45-μm filter membrane to determine the content of heavy metal ions in the filtrate. Removal rate E in percentage was calculated as follows:

$$\mathrm{Removal Rate }\left(E\right)=\frac{{\mathrm{C}}_{0}-{C}_{1}}{{C}_{0}}\times 100\; \mathrm{in percent }(\mathrm{\%})$$

where C0 is the concentration of heavy metal ions in the solution before adsorption (mg/L) and C1 represents the concentration of heavy metal ions in the solution after adsorption (mg/L).

4 Results and analysis

4.1 Surface morphology characterization of coconut shell activated carbon

As shown in Fig. 2 for the surface morphology, more prominent pores and irregular structures were observed in modified coconut shell-based activated carbon (AC-1 through AC-4) than natural carbon (AC-0). The pore structure in modified carbons is more complex and changeable. The shape of pores in both modified and natural carbons are mainly ellipses, but the observed pores are macropores with more complex pore structure.

4.2 The effect of pH on the adsorption of Cu2+, Cd2+, Pb2+, and Zn2+

To investigate the effect of different pH values of coconut carbons on the removal rate of heavy metals, pH value ranges from 2 to 9. The initial condition of the solution of Cd2+, Cu2+, Pb2+,, and Zn2+ are five mg/L concentration, 60 min of adsorption time, room temperature, pH = 5, and adsorbent additive quantity under the condition of 1 g. The results on absorption performance with removal rate are shown in Fig. 3.

The change of pH value in wastewater solution affects the removal rate of heavy metal ions in coconut shell activated carbon [14,15,16,17,18,19,20,21]. Different pH value affects the charge on the surface of activated carbon and electrostatic interaction between activated carbon and heavy metal ions and influences the heavy metal ions in aqueous solution.

As shown in graphs in Fig. 3, as the pH value in the aqueous solution increased, the removal rate of heavy metal ions gradually increased. This effective absorption comes from the slow dissociation of the functional group of H+ in coconut carbons and its more prolonged exposure to the absorption area. Around a pH value of 5, the removal rate of heavy metal ions by modified coconut carbons with nitric acid, sodium hydroxide, and potassium permanganate (AC-1 through AC-3) reached a peak of above 90%. With more pH in aqueous solution than pH 5, these modified coconut carbons’ adsorption capacity showed a decreasing trend. On the other hand, modified coconut carbons with iron salt (AC-4) showed its peak near pH = 7, and natural carbon (AC-0) showed an increase of removal rate as increased pH value.

4.3 Dosage of adsorbent of heavy metal ions

Under several initial conditions of concentrations of heavy metals (Cu2+, Cd2+, Pb2+, and Zn2+) in aqueous solution with 5 mg/L, 60 min of the adsorption time, room temperature of 25 °C, and the pH value = 5, the influence of the dosage of activated carbon on the removal effect of heavy metal ions was investigated with the range of 0.1–7 g and shown in Fig. 4.

As shown in Fig. 4, for a low dosage of activated carbon’s adsorbent (< 1 g), the adsorption rate of heavy metal ions modified by nitric acid, sodium hydroxide, and potassium hyper-menthol (AC-1 through 3) increases rapidly. And when it reaches about 1 g, these three modified carbons show more than 90% for the four heavy metals’ removal rate. For further increase of dosage, the adsorption rate seems to be saturated with a slow rise, which may be caused by the overlapped adsorption sites by the overcrowding of adsorbent particles. For the dosage of both iron-salt modified carbon (AC-4) and unmodified coconut shell activated carbon (AC-0), when it is less than 6 g, the adsorption rate of both was positively correlated with the dosage. At the dosage of 6 g of AC-0, the adsorption of both reached a balance, and the removal rate was above 73%.

In general, the increase of adsorbent dosage leads to the improved removal rate of the adsorbent to heavy metal ions. However, the rise of absorbent dosage is only one factor in absorption effects due to its saturation. Instead, the continued increase of the adsorbent amount will reduce the adsorption amount of the adsorbent per unit mass, resulting in the waste of the adsorbent.

4.4 Adsorption kinetics of heavy metals (Cu2+, Cd2+, Pb2+, and Zn2+) to different coconut shell activated carbons

Under the initial conditions of concentrations of Cu2+, Cd2+, Pb2+, and Zn2+ with 5 mg/L, the temperature of 25 °C, dosage of activated carbon of 1 g, and pH = 5, the influence of contact time (ranging from 5 to 240 min) on the adsorption effect of heavy metal ions with different types of coconut shell-based activated carbons were investigated. The relationship between reaction time and the removal rate of heavy metal ions within wastewater with different carbons is shown in Fig. 5.

Fig. 5
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Effect of agitation time on adsorption performance of different types of coconut shell activated carbons; ordered in AC-0 through AC-5

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The adsorption kinetic curves of coconut shell activated carbons for natural (AC-0) and modified (AC-1 through 4) show a relationship of the contact time between activated carbons and the removal capacity of heavy metal ions wastewater.

At the initial stage of adsorption, the driving force of diffusion of Cu2+, Cd2+, Pb2+, and Zn2+ in an aqueous solution was large enough to increase the absorption rate. As the adsorption increased continuously, the heavy metal ions spread into the holes of coconut shell activated carbons until those make adsorbents in the center. The removal rate was further improved, and its rate gradually decreased, and finally reached the equilibrium points with saturation.

During the first 60 min, the four heavy metal ions’ adsorption rate on the natural coconut shell activated carbon (AC-0) was faster. When the contact time exceeded 60 min, the adsorption rate of AC-0 decreased slowly. With the continuous increase of the contact time, the adsorption capacity gradually reached saturation. The adsorption and removal capacity of AC-0 on four heavy metal ions is significantly different. Its adsorption and removal effect on Pb2+ is the most, reaching 87%, while the removal rate of Cd2+ is only 44%.

The adsorption of four heavy metal ions in an aqueous solution with the modified carbons of acid (AC-1), alkali (AC-2), and potassium (AC-3) was settled to a balance point within 60 min. When the contact time was extended to 120 min, the adsorption with iron salt modified carbon (AC-4) was also saturated. After reaching adsorption equilibrium, the removal rates of the four heavy metal ions with modified carbons (AC-1, AC-2, and AC-3) got to 88%. Among them, the removal of Zn2+ by AC-1 and AC-3 was the highest with 95%, 96.5%, respectively. For removing Cd2+, AC-2 was the highest at 98%, and the adsorption effect of Pb2+ by AC-4 was the best at 92%.

Compared with the natural carbon (AC-0), the adsorption equilibrium time of modified carbons (AC-1 through AC-4) to heavy metal ions is shorter, and the removal effect is better. The modified coconut shell activated carbons provide more adsorption sites to have increased contacts between heavy metal ions and the adsorbent and result in rapid adsorption.

5 Conclusion

Compared with the natural coconut shell activated carbon (AC-0), modified carbons (AC-1 through AC-4) responded to heavy metal ions with a shorter response time and fast saturation and showed improved removal effect. Among different modifications, the adsorption capacity with iron-salt modified coconut shell activated carbon showed an inferior performance than the other three modifications with acid, alkali, and potassium permanganate. More than 90% of heavy metal ions in wastewater were absorbed and removed successfully by nitric acid (AC-1), sodium hydroxide (AC-2), and potassium high-mentholated (AC-3). The adsorption and removal effect of iron-salt modified carbon (AC-4) and natural carbon (AC-0) reach the best with 6 g of dosage. A reasonable increase in adsorbent dosage shows enhancing the adsorption and removal capacity to heavy metal ions in an aqueous solution. At the pH value of the aqueous solution with 5, the adsorption of heavy metal ions with three modified carbons (acid, alkali, and potassium modified with high menstruate) reached the optimal level. As the pH value increased to 7, iron-salt modified carbon’s adsorption effect reached the optimal level. In the weak acid condition, the binding ability of hydrogen ions to the carboxyl group is vital. With the increase of pH value in the solution, the ionization degree of hydroxyl groups increases. The possibility of heavy metal ions being adsorbed increases, so the removal rate increases rapidly. When the pH value is 5, the adsorption capacity of three kinds of modified coconut shell activated carbon (acid, alkali, and potassium permanganate) is the strongest. With the further increase of the solution’s pH value, the adsorption effect of the adsorbent for heavy metal ions showed a slight decrease, which may be caused by the gradual increase of chemical interaction between heavy metal ions and hydroxyl ions.

Table 1 Physical and chemical properties of coconut shell activated carbon
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