Introduction

Toxicity of toxic metals implies the unwanted concentration which is found naturally on earth. This becomes concentrated as a result of human activities, which enters into the soil to plant to animals then to human tissue through inhalation, ingestion, and dermal exposure through handling, and can bind to, and interfere with the functioning of the vital cellular component (Okereafor et al. 2020). Toxic metals are discharged into wastewater and are found to be carcinogenic hence causes a serious threat to human health and the environment (Chowdhary et al. 2020).

Wastewater contains contaminants that are extremely persistent and can change into an uncontrollable form which are harmful to human health and the environment (Adeola and Forbes. 2021). The worlds’ water quality is being depleted because of the unceasing discharge of huge amount of biological and other pollutants viz. dyes, surface-active agents, toxic metals (gold (Ag), cadmium (Cd), mercury (Hg), cyanide (Cn), arsenic (As), lead (Pb), chromium (Cr), among others), pharmaceuticals, pesticides, weedicides and herbicides (Aribam et al. 2021). Hence, to avoid these dangers it’s important to eliminate these toxic metals from the wastewater by using environmentally friendly materials for the adsorption.

Coagulation-flocculation, adsorption, membrane filtration, reverse osmosis, chemical precipitation, ion exchange, electrochemical treatment, solvent extraction, and flotation are just a few of the technologies used to remove inorganic contaminants around the world (Adekeye et al. 2019). These technologies suffer from a range of disadvantages stretching from inefficiency to removal of pollutants at low concentration and to completely convert pollutants into biodegradable or less toxic byproducts, high energy, and chemicals consumption, process complexity, high maintenance and operation costs, among others (Yang et al. 2019). Hence, there should be an effective and feasible action that should meet equally commercial and ecological necessities to be marketed and applied on large scale.

Adsorption is one of the easiest, cheapest, and less time-consuming methods of wastewater treatment which has been studied and researched by scientists (Crini and Lichtfouse. 2019). Agricultural waste is a resource for the removal of toxic metals via adsorption processes (Dai et al. 2018). Products from agricultural waste such as biochar has been proven to have a good removal rate of pollutants adsorption (Deng et al. 2017) and, the treatment procedure is ecologically approachable and cost-effective. New research is needed to integrate low-cost and readily available materials into various treatment processes in order to lower total treatment costs and improve process efficacy for the removal of hazardous metals.

There are limited studies on the cacao pod husk biomass utilisation which is low-cost material and environmentally friendly. Hence, the objective of the research was to assess the adsorption of toxic metals from aqueous phase using cacao pod husks. In the era of green chemistry, their utilisation can encourage development of national policies and legislation against cacao pod husk and other agricultural wastes burning to prevent and control pollution of air. This study provides a good foundation for effective utilisation of cacao pod husk waste for control of heavy metal pollution in water and soil.

Materials and methods

Cacao collection and biochar preparation

Cacao pod husks were collected from Kechebi in Nkwanta South Municipal in the Oti Region in Ghana. The cacao pod husks were washed with deionised water to remove particulate matter and filth that might interfere and/or interact with sorbed metal ions (Onawumi et al. 2020). The biomass was then air dried to remove moisture content to prevent the pods from degrading and decaying. Pods were broken into smaller sizes into an earthen pot and then transferred into a muffle furnace for pyrolysis under limited oxygen (O2) at a higher temperature of 500 ± 5 °C (fast pyrolysis) for 30 min. This was done continuously to obtain 503.50 g biochar. The biochar was crushed and sieved using a 2 mm sieve.

Preparation of stock solutions and aqueous solution for adsorption experiment

For the preparation of stock solutions; lead, mercury and cadmium were obtained by weighing 1.60 g of lead nitrate (Pb (NO3)2, 1.35 g of mercury chloride (HgCl2) and 1.68 g of cadmium chloride (CdCl2), respectively, to prepare the stock solution. The molecular weight of the Pb (NO3)2 (331.21), (HgCl2) (271.50) and (CdCl2) (183.32) were divided by the atomic weight of Pb (207.20), Hg (200.60) and Cd (122.41), respectively, to obtain 1 mg of each toxic metals. Toxic metal solutions were prepared in a 1000 mL volumetric flask. Serial dilution was done to obtain maximum contamination thresh holds limits for Pb2+, Hg2+ and Cd2+ (1.00 mg/L, 5.00 mg/L 10.00 mg/L and 20.00 mg/L), respectively.

Adsorption experiment

Cacao pod husk biochar were weighed using Sartorius Analytical Weighing Scale (CP 125 S) and then transferred into a well washed conical flask. A pipette was used to transfer the desired concentrations of the toxic metals (Pb2+, Hg2+ and Cd2+) from stock solution into 1000 mL volumetric flask of distilled water. 100 mL of the toxic metal was added to the biochar. The mixture was set up on a rotatory orbital shaker and agitated for 60 min (contact time) at 150 rpm. The pH and temperature were recorded using a pH meter. Whatman's qualitative filter paper with a particle retention size of 125 mm was used to filter samples into 40 mL plastic bottles via glass funnels. The experiment was also carried out under three different concentrations of each spiked Pb2+, Hg2+ and Cd2+ (1.00 mg/L, 5.00 mg/L, 10.00 mg/L, and 20.00 mg/L for all the metals, respectively). The elutes obtained were analysis for the Pb2+, Hg2+ and Cd2+. Using standard calibration curves, the absorbances recorded by the Atomic absorption spectrometry (AAS) were translated to concentrations.

Calculations for adsorption efficiency of lead, mercury and cadmium using cacao pod husk biochar

The adsorbent uptake for each of the toxic metal at equilibrium were computed using the symbol Qe (Dada et al. 2012). Hence, calculating the adsorption efficiency can be represented mathematically as Equ. (1):

$$\mathrm{Qe }=\frac{{{\varvec{C}}}_{{\varvec{i}}}-\boldsymbol{ }{{\varvec{C}}}_{{\varvec{f}}}}{\mathbf{M}}\boldsymbol{ }\times {\varvec{V}}$$
(1)

To determine, the adsorption capacity percentage, it’s also represented as:

$$\mathrm{Qe }= \frac{\left({{\varvec{C}}}_{{\varvec{i}}}-\boldsymbol{ }{{\varvec{C}}}_{{\varvec{f}}}\right)}{{{\varvec{C}}}_{{\varvec{i}}}}\boldsymbol{ }\times 100$$
(2)

where Qe is the adsorption capacity, Ci is the original concentration of the toxic metal, and Cf is the final concentration of toxic metals after adsorption.

Langmuir and Freundlich isotherms

The main isotherm models used in computing are Langmuir and Freundlich. This can be illustrated in a linear equation mathematically. Below is the equation formula provided for Langmuir isotherm;

$$\mathrm{Qe }= \frac{{{\varvec{Q}}}_{{\varvec{m}}{\varvec{a}}{\varvec{x}}{\mathbf{K}}_{{\varvec{L}}}{\mathbf{C}}_{{\varvec{e}}}}}{1+\boldsymbol{ }{\mathbf{K}}_{{\varvec{L}}}{\mathbf{C}}_{{\varvec{e}}}}$$
(3)

where Ce the concentration of the adsorbable at equilibrium (mg/g), KL is the Langmuir constant (L/mg) and Qmax (mg/g) is the amount of adsorbed molecules on the adsorbent surface at any time (Dada et al. 2012). RL can also be used as a separation factor. This will help us to better establish the important features of the Langmuir adsorption isotherm model. RL can be denoted as:

$$ R_{{\text{L}}} { } = { }\frac{1}{{1 + {\varvec{K}}_{{\varvec{l}}} {\varvec{C}}_{{\varvec{o}}} }} $$
(4)

Where KL is the Langmuir constant (mg/g), and Co is the adsorbate initial concentration.

When the RL > 1, the adsorption could be non-conducive. When the RL = 1, it is linear, when RL = 0, it is permanent and finally when 0 ˂ RL ˂ 1 is favourable (Dada et al. 2012).

The mathematical representation of the Freundlich adsorption isotherm form is:

$$\mathrm{Log qe }=\mathrm{ Log KF }+ \frac{1}{\mathbf{n}}\mathrm{log Ce}$$
(5)

where KF is the adsorption capacity (L mg−1) and \(\frac{1}{\mathbf{n}}\) is the intensity of the adsorption. The \(\frac{1}{{\varvec{n}}}\) shows how energy is relatively distributed and heterogeneous nature of the biochar surface (Dada et al. 2012). If \(\frac{1}{\mathbf{n}}\) ˂1, it means the adsorption is normal, if \(\frac{1}{\mathbf{n}}\) >1, it indicates that there is co-operative adsorption, and if n = 1, it means there are two-phase partition that does not rely on concentration to occur (Dada et al. 2012).

The standard Freundlich adsorption isotherm model is

$$\mathrm{Qe }= {\mathbf{K}}_{{\varvec{F}}}{{{\varvec{C}}}_{{\varvec{e}}}}^{\frac{1}{{\varvec{n}}}}$$
(6)

where Qe is the volume of toxic metal removed at equilibrium, per gram of the adsorbent (mg g−1), KF is the Freundlich isotherm constant (mg g−1), Ce is the concentration of adsorbable at equilibrium (mg L−1) and finally, \(\frac{1}{\mathbf{n}}\) is the adsorption intensity.

Results and discussion

Adsorption of lead onto biochar produced at 500 ± 5 °C

The adsorption capacity of cacao pod husk biochar for lead at different concentrations (1.00, 5.00, 10.00 and 20.00 mg/L) ranged from 97.00% to 99.40% (Table 1). Cacao pod husk biochar had a higher affinity for lead in the monosystem. Similar study by Yong et al. (2018) in Malaysia used cacao pod husk to produce biochar at pyrolysis of 500 °C to adsorb lead from an aqueous solution with optimising parameters showed that lead ions had adsorbed onto the adsorbent through a monodentate coordination bonding with carboxylate groups. Zhang et al. (2018) found that biochar has a wide specific surface region, porous structure, abundant surface functional groups and mineral components, making it a good adsorbent for removing contaminants from aqueous solutions specifically toxic metals.

Table 1 Adsorption efficiency of the toxic metals by cacao pod husk biochar
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Other agricultural raw material has been used to adsorb toxic metals from wastewater and aqueous solution. Also, Duwiejuah (2017) used shea nut shell and ground nut shell biochar to adsorb lead from the aqueous phase in a monosystem, the removal efficiency was higher than 99.10%.

Adsorption of mercury onto biochar produced at 500 ± 5 °C

The adsorption efficiency of cacao pod husk biochar for mercury at different concentrations (1.00, 5.00, 10.00 and 20.00 mg/L) ranged from 99.97% to 99.99% (Table 1). Cacao pod husk biochar had a higher affinity for mercury in the monosystem. An increase in dosage of the cacao pod husk biochar with a respective increase in the concentration showed complete adsorption. The reason being that an increase in adsorbent has a corresponding increase in the number of active surfaces for ions interaction between adsorbent and adsorbate. Similar study by Liu et al. (2016) reported on how biochars produced from different biomass were used to reduce mercury concentration from river water by 99.80%. Also, Park et al. (2019) using a combination of sulfurised wood biochar and wood biochar to efficiently remove mercury from the aqueous phase. It is clear that biochar which is economically effective, easy to use, environmentally friendly has a strong and good adsorption efficiency.

Adsorption of cadmium onto biochar produced at 500 ± 5 ˚C

The adsorption efficiency of cacao pod husk biochar as an adsorbent for cadmium at different concentrations (1.00, 5.00, 10.00 and 20.00 mg/L) ranged from 99.85% to 99.99% (Table 1). This study proves that cacao pod husk biochar produced under the fast pyrolysis temperatures had the strongest affinity for cadmium in the monosystem. Cacao pod husk biochar demonstrated good features to be used as a percussor in biochar production due to its cellulosic and hemicellulosic content (Oladayo 2010). A similar test of removing cadmium from aqueous solution using Algerian cork powder as an adsorbent with a maximum pH of 6 showed a removal efficiency of 64.48% (Krika et al. 2016). The maximum removal efficiency of activated carbon produced using palm kernel shell for cadmium within 30 min ranged from 93 to 94% but increased to 96% to 97% as contact time was 150 min (Faisal et al. 2019). The adsorption efficiency of cadmium was a result of the prolonged contact time. However, Rachmat et al. (2018) assessed the morphology of cacao pod husk, carbonised at 300 °C for 3 h (activated using 2 M HCl at 80 °C for 4 h) and realised that, it has a rough and heterogeneous surface with unequal distributed pores which made it efficient for adsorption.

The removal efficiency of the cacao pod husk as an adsorbent for toxic metals was above 90%. Hence, cacao pod husk biochar is a very good precursor for the development of adsorbents from the aqueous phase (Eletta et al. 2020). Biochar produced using cacao pod husk under fast pyrolysis proved a good removal efficiency of Cd in a monosystem. The current study proves that cacao pod husk biochar has had a strong and good affinity for the adsorption of toxic metals in the monosystem.

Effect of pH on the adsorption capacity of cacao pod biochar

The pH values of the toxic metals in monosystem are presented in Table 2. The adsorption of toxic metals is influenced by pH owning to the surface charge of the biochar, dissolution of mineral components and beingness of toxic metal ions (Wang et al. 2014). The effect of pH on adsorption for the most part depends on the type of biochar and the metal ion in point. The biochar’s surface charge is not only affected by pH but the degree of ionisation and speciation of the toxic metal (Kołodyńska et al. 2012). Biochar comprises distinct surface functional groups which change as the correlation increases in the pH of the aqueous phase. An increase in pH contributes to insoluble hydrated oxide or hydroxide precipitation, thereby lowering the ready accessibility of heavy metal ions for sorption, again, a decrement of pH results in an increment in the hydrogen ion concentration and a likely competition for the binding sites (Kahraman et al. 2008). Li et al. (2018) also reported that under acidic conditions the adsorption capacity of the adsorbent for Pb2+ and Cd2+ increase whilst the pH increases, the negative charge surface of the biochar and the acid dissociation degree of the organic functional increases thereby providing a conducive condition for the hydrolysis of the toxic metal.

Table 2 pH of the monometal solution
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Langmuir adsorption isotherm

Langmuir adsorption isotherm was employed in this experiment to estimate the adsorption capacity of the cacao pod husk biochar for the toxic metals. Qmax (mg/g) is the maximum adsorption capacity for monometals, for Pb2+ was -1.97 × 10–5 mg/g, Hg2+ was 1.01 × 106 and Cd2+ was 2.10 × 106 (Table 3). RL for Pb2+ was 0.58, Hg2+ was 1.00 and Cd2+ was 1.00 in the monosystem (Table 3). The correlation coefficient (R2) values for Langmuir adsorption isotherm for lead, mercury and cadmium in the monosystems are 0.61, 0.63 and 0.07, respectively (Figs. 1, 2 and 3).

Table 3 Adsorption isotherm modelling the results of the toxic metals of the monosystem
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Fig. 1
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Langmuir isotherm graph for the adsorption of lead onto cacao pod husk biochar

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Fig. 2
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Langmuir isotherm graph for the adsorption of mercury onto cacao pod husk biochar

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Fig. 3
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Langmuir isotherm graph for the adsorption of cadmium onto cacao pod husk biochar

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Langmuir isotherm was employed for the adsorption of toxic metals in the monosystem with an identical and finite number of sites (Al-Ghouti and Da'ana 2020). The adsorbate's interaction with the adsorbent is represented by the Langmuir constant. The maximum adsorption capacity of lead was − 1.96 × 105 mg/g, which is significantly less than the 220.94 mg/g maximum adsorption capacity of Pb2+ from Caragana korshinskii biomass used as biochar (Wang et al. 2021). The bigger the value of the constant, the stronger the interaction between the adsorbate and the adsorbent, whilst the smaller the value, the weaker the connection (Tran et al. 2019). From the results, lead value for KL was − 0.04 which indicates a weak interaction. The RL value for the lead was 0.58 implying favourable. Langmuir Isotherm model best fit for the lead adsorption experiment. The greatest adsorption capacity of mercury was 1.00 × 106 mg/g. The Langmuir constant reflects how much the adsorbate interacts with the adsorbent. A bigger constant value suggests a strong association between the adsorbate and the adsorbent, whereas a smaller value indicates a weak interaction (Tran et al. 2019). From the results, the value of KL for mercury was 0.06 which indicates a weak interaction. The RL value for mercury was 1.00 implying it was linear. The R2 value derived from the graph which was the correlation coefficient is 0.63. Cadmium showed a maximum adsorption capacity of 2.10 × 106 mg/g which is far lower than the maximum adsorption capacity of 42.43 mg/g from Caragana korshinskii biomass used as biochar (Wang et al. 2021). The Langmuir constant indicates the extent of interaction between the adsorbate and the adsorbent. However, if the value of the constant is larger that indicates a strong interaction between the adsorbate and adsorbent whilst a smaller value implies a weak interaction (Tran et al. 2019). From the results, cadmium the value for KL was 0.00 which indicates a weak interaction. The RL for cadmium was 1.00 which implies linear. A similar study by Meroufel et al. (2013) observed RL for cadmium was also linear.

Freundlich adsorption isotherm

Freundlich adsorption isotherm was employed to consider the fitness of the monoexperiments with the heterogeneousness of surface sites of the cacao pod husk biochar adsorbent and the adsorption of toxic metals. The 1/n for the toxic metals in the monosystems was as follows, for Pb2+, Hg2+ and Cd2+ was − 1.76, 0.90, and 1.62, respectively (Table 3). KF (mg/g) for Pb2+, Hg2+ and Cd2+ were 3.26, 7.08 and 1.31, respectively (Table 3). Freundlich adsorption isotherm correlation coefficient (R2) values for Pb2+, Hg2+ and Cd2+ are 0.51, 0.85 and 0.12, respectively (Figs. 4, 5 and 6). The Freundlich adsorption isotherm was used to describe the adsorption process between the toxic metals under study (Cd2+, Hg2+ and Pb2+) and the cacao pod husk biochar. Freundlich isotherm provides an expression that permits the explanation of heterogeneous surfaces of adsorbent and the exponentially distributed actives sites on them and their energies (Kumar et al. 2019). The KF (Freundlich constant) was 3.26. The 1/n value for Pb2+ was − 1.76 in the monosystems which was below unity signifying that the adsorption was favourable with the heterogeneous surfaces. The N value for Pb2+ was − 0.57 and the R2 value for Pb2+ was 0.51. The Freundlich isotherm results for mercury, where KF was 7.08. The 1/n value for Hg2+ was 0.90 in the monosystems was below unity signifying that the adsorption was favourable with the heterogeneous surfaces. The N value for Hg2+ was 1.11 and the R2 value for Hg2+ was 0.85. Freundlich isotherm model best fit for the experiment in terms of mercury adsorption.

Fig. 4
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Freundlich isotherm graph for the adsorption of lead onto cacao pod husk biochar

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Fig. 5
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Freundlich isotherm graph for the adsorption of mercury onto cacao pod husk biochar

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Fig. 6
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Freundlich isotherm graph for the adsorption of cadmium onto cacao pod husk biochar

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The Freundlich isotherm results for cadmium, KF was 1.31. The 1/n value for Cd2+ was 1.62 in the monosystems was above unity signifying that the adsorption by the adsorbent is favourable with the heterogeneous surfaces. A similar work done by Meroufel et al. (2013) reported 1/n value of 1.30. The N value for Cd2+ was 0.62 and R2 value of 0.62 in this study whereas 0.9087 for Cd2+ was reported by Meroufel et al. (2013). In terms of cadmium adsorption, the Freundlich isotherm model fits the experiment best (Figs. 4, 5 and 6).

Conclusion

The removal efficiency of the three toxic metals by the cacao pod husk as an adsorbent proved to be very efficient. Cacao pod husk is the raw material obtained from agricultural waste (environmentally friendly and cost-effective) which was used to produce biochar, at a temperature of 500 ± 5 °C, has a good and strong affinity for Pb2+, Hg2+ and Cd2+ for the adsorption of toxic metals from the aqueous phase. The biochar produce at fast pyrolysis has a strong and good affinity for Pb2+, Hg2+ and Cd2+in the monophase. The temperature, contact and rotary time, dosage of adsorbent and pH of the solution have an impact on the effective adsorption. Freundlich isotherm model favour’s mercury and cadmium adsorption in the experiment of whilst the Langmuir model favour’s for the adsorption of lead. Further research should be conducted using the cacao pod husk biochar to assess the adsorption efficiency in the binary and ternary systems.