**4. Groundnut shells in heavy metal adsorption**

Groundnut shell is an abundant agro-industrial waste product that is mainly obtained after the removal of groundnut seed from its pod. Groundnut shell is rich in lignin content, due to which it undergoes very slow degradation [41]. Groundnut shell plays a vital role in the adsorption of heavy metals from water sources. Groundnut shell contains cellulose, organic acids, lignin, pentosan, and tannins which aid in binding heavy metal ions [42]. Efficiency of groundnut shells in heavy metal removal from water sources has been estimated by various researchers. For example, in an experimental study, the efficacy of groundnut shells for removing heavy metals from wastewater was tested by using several parameters at a standard temperature, such as the effect of pH, contact time, and dosage of groundnut shell. It has been found that the removal efficiency of groundnut shells for copper and lead is 68.19 and 77.81%, respectively [43].

Groundnut shell has been proven to be an efficient and effective adsorbent for the removal of toxic metals from zinc and chromium-plated water in which the initial concentration of zinc and chromium has been observed to reduce to the values 0.75 and 0.85, respectively. It has been concluded with the manifestation that the ability to remove toxic metals from zinc and chromium-plated water can be effectively achieved by using a groundnut shell as the adsorbent [44].

Groundnut shell has been utilized for removing copper from a water source, using batch experiments and considering different parameters, such as pH, contact time, initial concentration of metal ion, and dosage. The efficiency of groundnut shells at pH 6 for removing copper metal from water sources has been found to be approximately 94% [45]. Groundnut shell-activated carbon can also be used for the adsorption of metal ions such as Cu (II), Zn (II), Ni (II), and Cr (IV) from the aqueous solution at different parameters, such as pH and metal ion concentration of the aqueous solution. It has been estimated that the groundnut shell activated carbon has the highest adsorption capacity for removing chromium ions than its adsorption capacity for other metal ions from water sources [46].

Groundnut shells can also be positively utilized for the removal of lead ions from the aqueous solution. The adsorption capacity of groundnut shells for lead ions has been estimated at nearly 39 mg/g. Significant removal of lead ions by groundnut shells from water sources has been observed at different temperature conditions and different pH values [18]. The maximum adsorption capacity of groundnut shells at low pH values for chromium (VI) has been estimated at around 4.3 mg/g [25].

The effective removal of chromium(III) and copper(II) using groundnut shells has been evaluated at the maximum adsorption capacities of 27.89 and 25.39 mg g-1, respectively [32]. Heavy metal removal by using groundnut husk has also been significantly observed with maximum adsorption capacities of 7.69, 10.21, and 29.11 mg g-1 for Cr(III), Cu(II), and Pb(II), respectively [47]. The maximum adsorption capacity of groundnut shells for copper, nickel, and zinc has been calculated as 5, 8.1, and11 mg/g, respectively [48]. These adsorption capacities have been increased by 40–70% by using reactive dye in chemically modifying the groundnut shells using [48].

Groundnut shells can be effectively used as a raw material for preparing activated carbon to remove chromium and nickel from dye effluents based on the contact time. It has been observed that groundnut shell-activated carbon leads to an increase in chromium and nickel adsorption with an increase in contact time [49].

The adsorption of heavy metals, such as copper, zinc, lead, manganese, and cadmium, from wastewater by groundnut shells has been observed at a concentration *Removal of Heavy Metals from Wastewater with Special Reference to Groundnut Shells… DOI: http://dx.doi.org/10.5772/intechopen.109904*

### **Figure 1.**

*Mn(II) and Pb(II) adsorption by using groundnut shells at different concentrations [50].*

range of 10–50 mg/100 ml [50]. It has been evaluated that at a contact time of 100 min, maximum adsorption of groundnut shells for lead and manganese can be found. Manganese(II) and lead(II) adsorption from wastewater by using groundnut shells at different concentrations has also been observed at different concentrations at 33°C for 90 mins (**Figure 1**).

From the **Figure 1**, it is clearly observed that the adsorption of Mn(II) and Pb(II) increases with the increase in the concentration of groundnut shells [50].

### **5. Groundnut shells derived biochar in heavy metal adsorption**

Groundnut shell-derived biochar has been used as an alternative cost-effective adsorbent to remove various heavy metals from water sources. The surface of groundnut shell biochar possesses functional groups, such as COOH and OH [51, 52]. Biochar with high efficiency can be derived from groundnut shell since no pre-treatment of biochar surfaces for adsorption is needed [53]. The production of biochar from groundnut shells by pyrolysis has been considered an eco-friendly and efficient method for the disposal of agricultural solid waste [54]. Groundnut shell biochar produced under slow and fast pyrolysis temperatures has been observed to possess a strong affinity for the removal of heavy metals from water sources. Various research experiments have been carried out to determine the adsorption efficiency of groundnut shell-derived char for different heavy metals at different parameters, such as pH of the aqueous solution, constant biochar dosage, particle size, concentration levels of heavy metal ions, constant contact time, and temperature [53].

Groundnut shell-derived biochar has been utilized for the removal of heavy metal ions from the aqueous solution at 350 ± 5°C and 700 ± 5°C. The adsorption efficiency of groundnut shell biochar for the removal of cadmium ions from the aqueous solution with concentration levels of 0.04, 0.08, and 0.20 mg/l has been estimated at 100, 99.99 and 100% at pyrolysis temperature of 350 ± 5°C, respectively. The adsorption efficiency of groundnut shell biochar for cadmium with the same concentration levels at a pyrolysis temperature of 700 ± 5°C has been estimated 100, 99.67, and 99.93%, respectively (**Table 3**). Groundnut shell biochar adsorption efficiency has been found

approximately greater than 99.60% for cadmium [53]. A similar research study has reported 99.2% cadmium removal efficiency from wastewater at room temperature by using 40 g/l of adsorbent dosage, 200 mg/l of initial concentration of cadmium and pH 5 [55].

The adsorption efficiency of lead ions has been calculated 99.12% at the concentration of 0.10 mg/l, 100% each at 0.20 and 0.50 mg/l at pyrolysis temperature 350 ± 5°C, respectively, while the adsorption efficiency of lead ions in aqueous solution with same concentrations has been calculated 100% at pyrolysis temperature 700 ± 5°C was 100% (**Table 3**). The removal efficiency of lead ions in a mono-component system by groundnut biochar has been found to be higher than 99.1% [53].

The removal efficiency of mercury ions in the aqueous solution with a concentration of 0.10, 0.20, and 0.50 mg/l at 350 ± 5°C has been calculated 99.99, 100 and 100%, respectively (**Table 3**), while removal efficiency of mercury by groundnut shell biochar with same concentration levels of mercury ions has been estimated 100% at high pyrolysis temperature (**Table 3**). The removal efficiency of Hg2+ by low and high pyrolysis temperatures in mono-component systems with different maximum contamination limits has been reported to be almost 100% [53].

It has been found that groundnut shell-derived biochar at 400°C exhibits the highest capability for the removal of heavy metals from water sources because of its high specific surface area and rich functional groups [54]. The physicochemical properties of groundnut shell biochar prepared at varying pyrolysis temperature is shown in **Table 4**. With the temperature elevation from 350 to 400°C, the surface area and total pore volume has been observed to increase from 3.77 to 6.45 m<sup>2</sup> /g and from 0.0097 to 0.0161 cm3 /g, respectively [54]. However, as the temperature increases more than 400°C, surface area and pore volume have been found to decrease. This phenomenon has been attributed to too-high temperatures that accelerated the deformation and collapse of micropores in biochar. Therefore, groundnut shell biochar at 400°C has the highest surface area and pore volume [56]. With the increase in temperature from 350 to 600°C, the pH value of the produced biochar has been observed to increase from 9.11 to 10.35, and the biochar yield has declined from 47.9 to 33.6%. The rise in pH has been observed by the decomposition of organic acid and carbonate into alkali


### **Table 3.**

*Adsorption efficiency of groundnut shell biochar for Cd2+, Pb2+, Hg2+ ions [53].*

*Removal of Heavy Metals from Wastewater with Special Reference to Groundnut Shells… DOI: http://dx.doi.org/10.5772/intechopen.109904*


**Table 4.**

*The physicochemical properties of groundnut shell biochar prepared at varying pyrolysis temperatures [54].*

salts. Therefore, it is beneficial for the heavy metal ions to precipitate in an alkaline environment [54].

The initial concentration of the metal ions in an aqueous solution has been considered to play an important role in the removal of metal ions in aqueous solution by the groundnut shell biochar. For example., the removal of Pb(II) ions in an aqueous solution has been reported to decrease with the increasing initial concentration of Pb(II) ions from 100 to 400 mg/l [57]. The experimental results are shown in **Figure 2**.

Similarly, pH of the solution is considered as one of the most important parameters in the removal of heavy metals from aqueous solutions. For example, the effect of pH on the adsorption of Pb(II) ions on the biochar derived from the groundnut shell has been carried out by changing the initial pH of the solution from 2 to 6. The removal of Pb(II) ions has been evaluated to increase with the increasing pH. The removal of Pb(II) ions by the groundnut shell biochar has been found to be very low at a pH value of 2.0. The increase in pH has been reported to promote the removal of heavy metals [54]. The effect of pH on the removal of Pb(II) ions from the aqueous solution by groundnut shell biochar is shown in **Figure 3**.

From **Figure 3**, it can be concluded that the pH plays a vital role in the removal of Pb(II) ions from the aqueous solution by groundnut shell-derived biochar. With an

### **Figure 2.**

*Effect of initial concentration of the Pb(II) ions in aqueous solution on the removal of lead ions by the groundnut shell biochar.*

**Figure 3.** *Effect of pH on the removal of Pb(II) ions from the aqueous solution by groundnut shell biochar [54].*

increase in the pH value, the covered H3O+ leaves the biochar surface and makes the sites available to Pb(II) ions [57]. Some studies have also reported that pH-dependent surface charges play a crucial role in controlling the surface adsorption of heavy metal ions through electrostatic interactions [58].

### **6. Conclusions**

In developing countries, the increase in water scarcity and pollution significantly leads to less accessibility to clean drinking water. Heavy metal contamination in drinking water sources is a growing concern in the present era. Moreover, there is a lack of wastewater treatment methods in developing countries that would remove heavy metals from wastewater. As a result, various research studies have been conducted to investigate the use of low-cost adsorbents to remove heavy metals from water sources.

Groundnut shell is generally considered an agro-industrial waste; millions of tons of its quantity are produced every year as a leftover. Groundnut shell is rich in lignin, due to which these shells undergo slow degradation in a natural environment. Groundnut shell biomass has a wide range of applications. Groundnut shells can be converted into a valuable bio-product that can be efficiently and cost-effectively utilized in heavy metal removal from water sources. The chapter highlights the results of several scientific studies illustrating the adsorption efficiency of groundnut shells and biochar derived from the shell for the removal of various heavy metals from wastewater.

Various research studies have reported that groundnut shells can effectively remove heavy metals from water sources. The effectiveness of groundnut shells and biochar produced from groundnut shells at removing heavy metals depends on various parameters, such as pH, ionic strength, temperature, natural organic matter, initial concentration of heavy metal ions, etc. Groundnut shell-derived biochar *Removal of Heavy Metals from Wastewater with Special Reference to Groundnut Shells… DOI: http://dx.doi.org/10.5772/intechopen.109904*

exhibits a strong affinity for heavy metals in water sources at slow and fast pyrolysis temperatures.
