**7. Activated carbon from macadamia nut shells**

Several studies have addressed the use of large volumes of shell (mesocarp) that represents 40% of the dry weight of the fruit. In Paraguay, the potential of the shell of the *M. integrifolia* nut was studied as a precursor for obtaining activated carbon, whose volume in a macadamia processing industry is important. Activated carbon is the name applied to a series of porous carbon, which, after undergoing a carbonization and activation process, show great porosity and internal surface. It has been shown that activated is an extremely versatile adsorbent, with a crystalline structure similar to that of graphite, a characteristic that, together with the chemical nature of the carbon atoms that make it up, gives it surface adsorbent properties for a certain type of molecules [79]. The obtaining process basically consists of two stages: the pyrolysis of the carbonaceous material at temperatures below 800°C and the activation of carbon by physical or chemical methods.



*8.*

*Challenges and Advances in the Production of Export-Quality Macadamia and Its Integral Use… DOI: http://dx.doi.org/10.5772/intechopen.105000*

#### **Figure 9.**

*Fixed bed reactor for obtaining activated carbon: (1) cooling water inlet/outlet, (2) condenser, (3) burner, (4) collector of liquid byproducts, (5) fixed bed reactor, (6) basket with macadamia shells, (7) pyrolysis oven, and (8) pipe for steam injection. Elaboration: Smidt, M.*

The physical activation process was studied with a fixed bed reactor, consisting of a reaction chamber that contains a basket built with a metal mesh that supports the material to be carbonized (**Figure 9**). The reactor was placed in an electric furnace that allows the temperature to be controlled up to 1200°C; the steam necessary for activation was supplied by pumping liquid water with a peristaltic pump (not drawn) through a pipe inside the furnace, which allows the vaporization of this water. The gases produced are conducted out of the reactor, and the condensable fraction changes phase thanks to the removal of the heat in an indirect condenser through which cold cooling water circulates, while the incondensable gases are directed to a burner to burn the flammable fraction. About 750 g of macadamia nut shell (granulometry <1 mm) was introduced into the reactor and subjected to a heating rate of

#### **Figure 10.**

*Behavior of the iodine value (a) and yield of activated carbon (b) with temperature and activation time obtained from macadamia shells.*

*Challenges and Advances in the Production of Export-Quality Macadamia and Its Integral Use… DOI: http://dx.doi.org/10.5772/intechopen.105000*

19°C/min up to 400°C and then 7°C/min until reaching the established temperature. The effect of temperature (950°C and 1000°C) and activation time (20 and 30 min) on the yield of activated carbon and the absorption capacity, evaluated by the iodine value, was studied using a 2<sup>2</sup> factorial design. The flow rate of water injected for activation was 42 mL/min. **Figure 10** shows the behavior of the absorption capacity and the yield obtained in the experimental region. For the case of **Figure 10a**, it is noted that the highest absorption capacity was obtained at 950°C and 30 min. However, higher yields are obtained at the same temperature, but with 20 min of reaction (**Figure 10b**) and with an average difference in the absorption capacity of only 96 mg I/g. The analysis of variance with a 95% confidence interval revealed that, for both the independent variables, the temperature and the activation time are significant. However, the interaction between these variables is only significant for the yield.

### **8. Use of the shell as nanosorbent in the chemical industry**

Adsorption is a commonly used unitary operation for wastewater treatment due to its simplicity, cost-effectiveness, high removal capacity, and low energy consumption in large-scale processes [80]. Agricultural residues have the potential to be used as adsorbents due to their high availability and chemical composition [81]. Various byproducts of agricultural materials derived from adsorbents such as cactus leaves, hazelnut shells, banana peels, wool, almond shells, and coconut shells have been studied as potential adsorbents or contaminant removal from wastewater [82, 83]. Activated carbon is an adsorbent with a microporous structure, usually used for the elimination of polluting substances in wastewater, gases, etc. The microstructure of activated carbons depends not only on the natural texture of each raw material, but also on the activation process. Surface and porous areas are improved by carbonization and activation treatment. Improvements can be added by designing or modifying the activation process, such as carbonization at high temperature or in different atmospheres. Like other plant materials, the main component of the macadamia nut shell is cellulose (41.2%), which can be denatured to become activated [84]. Macadamia shell contains less inorganic content and high fixed carbon content compared to other biomass. Macadamia shells have a higher surface area than other nut shells [85, 86], and their ash content is very low [87]. Research has been conducted using macadamia nut shells to produce activated carbons for the removal of contaminants in water such as microcystin, aurocyanide, phenol, and methylene blue [88–91].

#### **9. Conclusion**

The fruits of macadamia nuts have wide advantages in their integral use, considering that the byproducts of the fruit are 100% industrializable, for which they present competitive advantages over other nontraditional crops. Several efforts have been made to improve the quality of nuts in countries such as Paraguay, with a tropical climate of high average temperatures and high relative humidity, which allow the macadamia nut to be a product of high nutritional and sensory quality, with a potential for sustainable and environmentally friendly production, compatible with current demands, within the framework of sustainable food for the future.
