Toxic Metal Adsorption from Aqueous Solution by Activated Biochars Produced from Macadamia Nutshell Waste

Abundantly available biomass wastes from agriculture can serve as effective environmental remediation materials. In this study, activated biochar was fabricated from macadamia nutshell (MCN) through carbonization and chemical modification. The resultant biochars were used as adsorbents to remove toxic metal ions such as Cu2+ and Zn2+ from aqueous solutions. The results showed that the activated MCN biochar has a high adsorption capacity for toxic metal ions. When MCN biochar was activated with K2CO3, the adsorption efficiencies for Cu2+ and Zn2+ were 84.02% and 53.42%, respectively. With H3PO4 activation, the Cu2+and Zn2+-adsorption performances were 95.92% and 67.41%, respectively. H2O2-modified MCN biochar had reasonable Cu2+and Zn2+-adsorption efficiencies of 79.33% and 64.52%, respectively. The effects of pH, adsorbent concentration and adsorption time on the removal performances of Cu2+ and Zn2+ in aqueous solution were evaluated. The results exhibited that the activated MCN biochar showed quick adsorption ability with an optimal pH of 4 and 4.5 for both Cu2+ and Zn2+, respectively.


Introduction
The rapid and continuous growth of industrialization and urbanization has caused serious environmental problems. Of these, contamination of water sources with toxic metals has attracted considerable interest from scientists and governments worldwide. Toxic metal ions-even at trace levels-can be extremely harmful to human health and ecosystems [1,2]. Therefore, the removal of toxic metal ions from aqueous solutions requires urgent attention. Many techniques have been effectively used for the removal of toxic metals ions from polluted wastewater, including, but not limited to, precipitation, use of filtration membranes, chemical treatment, reverse osmosis, electrochemical treatment and adsorption [3][4][5]. Among these methods, adsorption has been demonstrated as an effective pathway for the removal of toxic ions from contaminated water because of its low operation cost, high performance and simplicity.

Fabrication of Modified MCN Biochar from Macadamia Nutshells
Macadamia nutshells were collected as biomass waste and then washed thoroughly and dried before the carbonization process. Typically, MCN nutshells with uniform size were cleaned and thoroughly rinsed with distilled water, then dried at a temperature of 110 • C for 48 h. After primary treatment, the MCN nutshells were calcined at a temperature of 350 • C for 1 h with a heating rate of 23 • C per minute to form MCN charcoal. K 2 CO 3 -modified MCN biochar: MCN charcoal was immersed and agitated in K 2 CO 3 solution with a charcoal:K 2 CO 3 :water ratio of 1:1:10 for 24 h. The precipitate was filtered and dried at 110 • C for 24 h. The K 2 CO 3 -modified MCN charcoal was then carbonized in the furnace for 1 h at a temperature of 650 • C. The obtained samples were washed thoroughly with distilled water until the pH reached 7 and then dried at 110 • C. Samples were ground to fine particles and stored in a vacuum for further characterization.
H 3 PO 4 -modified MCN biochar: MCN charcoal was immersed and agitated in H 3 PO 4 solution for 24 h with a charcoal:H 3 PO 4 :water ratio of 1:1:10. The precipitate was filtered and dried at 170 • C for 1 h. The K 2 CO 3 -modified MCN charcoal was then carbonized in the furnace for 1 h at a temperature of 500 • C. The obtained samples were washed thoroughly with distilled water until the pH reached 7, then they were dried at 110 • C. Samples were ground to fine particles and stored in a vacuum for further characterization.
H 2 O 2 -modified MCN biochar: MCN charcoal was immersed and agitated in H 2 O 2 25% solution continuously for 48 h with charcoal:H 2 O 2 25% ratio of 1:10. After modification, the samples were washed with distilled water until neutral pH and dried at a temperature of 110 • C. Samples were ground to fine particles and stored in a vacuum for further characterization.

Toxic Metal Adsorption Studies
Effect of pH of the solution: The toxic metal ions employed in this study were Cu 2+ and Zn 2+ with a concentration of 30 ppm each. In the typical experiment, the modified-MCN biochar with a concentration of 0.3 g/L was added to 50 mL of Cu 2+ or Zn 2+ 30 ppm with the pH of the solution adjusted from 2.5 to 5.5. The adsorption time was 1 h. The precipitates were separated, and the residual was used to measure the remaining ions in the solution. The experiments were repeated three times.
Effect of biochar content: The modified-MCN biochar with the concentration ranging from 0.2 g/L to 2 g/L was added to 50 mL of Cu 2+ or Zn 2+ 30 ppm with the pH of the solution of 5.5. The adsorption time was 1 h. The precipitates were separated, and the residual was used to measure the remaining ions in the solution. The experiments were repeated three times.
Effect of adsorption time (adsorption kinetics): The modified-MCN biochar with the concentration of 0.3 g/L was added to 50 mL of Cu 2+ or Zn 2+ 30 ppm with the pH of the solution of 5.5. The adsorption time was from 0 min to 120 min. The precipitates were separated, and the residual was used to measure the remaining ions in the solution. The experiments were repeated three times.

Characterization of Biochars
The infrared absorption spectrum determines the FT-IR molecular functional group using the Perkin Elmer spectrophotometer with a resolution of 2 cm −1 and 16 scans (PerkinElmer, Inc., Waltham, MA, USA). All the spectra were recorded in the transmittance mode [33]. SEM particle size measurement and surface observation were conducted using a scanning electron microscope (SEM; JEOL, Ltd., Tokyo, Japan) at 2.0 kV and 10 µA. The samples were coated with gold powder before the images were captured. pH was measured directly using a pH meter (Mettler Toledo-S220K, Mettler Toledo, Greifensee, Switzerland).

SEM Image and FTIR Spectrum of the Modified-MCN Biochar
The morphology of the prepared modified-MCN biochar was observed using SEM and the result is shown in Figure 1a. It is evident that the prepared modified-MCN biochar has a porous microstructure with an average pore size of 10 µm. The appearance of the pore structures is due to the etching caused by the activating agent such as H 3 PO 4 as well as the activating condition at high temperatures. The chemical surface properties of the resultant biochar were investigated using an FTIR spectrum (Figure 1b). In the FTIR spectrum, the appearance of the vibration bands at 700 cm −1 and 400 cm −1 represents the stretching oscillation of the C=C functional group, which indicates that the C content increases in the biochar [34]. The vibration band at the wavelength of 3426.4 cm −1 is ascribed to the OH − stretching in the functional hydroxyl group, which is favorable for the metal ion adsorption [35]. The functional carbonyl groups (C-O and C=O) on the surface of biochar are also observed in the wavelength range of 1000 cm −1 to 2000 cm −1 of the FTIR spectrum, which indicates that the surface of the MCN biochar was successfully modified with the functional groups thereby improving the adsorption capability of the MCN biochar. The surface area of the modified-MCN biochar was determined to be 339,262 m 2 /g, which is reasonable for adsorption application.

SEM Image and FTIR Spectrum of the Modified-MCN Biochar
The morphology of the prepared modified-MCN biochar was observed using SEM and the result is shown in Figure 1a. It is evident that the prepared modified-MCN biochar has a porous microstructure with an average pore size of 10 µm. The appearance of the pore structures is due to the etching caused by the activating agent such as H3PO4 as well as the activating condition at high temperatures. The chemical surface properties of the resultant biochar were investigated using an FTIR spectrum (Figure 1b). In the FTIR spectrum, the appearance of the vibration bands at 700 cm −1 and 400 cm −1 represents the stretching oscillation of the C=C functional group, which indicates that the C content increases in the biochar [34]. The vibration band at the wavelength of 3426.4 cm −1 is ascribed to the OH − stretching in the functional hydroxyl group, which is favorable for the metal ion adsorption [35]. The functional carbonyl groups (C-O and C=O) on the surface of biochar are also observed in the wavelength range of 1000 cm −1 to 2000 cm −1 of the FTIR spectrum, which indicates that the surface of the MCN biochar was successfully modified with the functional groups thereby improving the adsorption capability of the MCN biochar. The surface area of the modified-MCN biochar was determined to be 339,262 m 2 /g, which is reasonable for adsorption application.

Adsorption of Cu 2+ by Modified-MCN Biochar
It is well-known that the pH of the solution plays a significant role in the adsorption behavior of the Cu 2+ , which is related to the dissolution and precipitation of copper [36]. With a pH of less than 6, copper is mostly present in the aqueous solution as ions, however, when the pH of the solution >6, copper ions tend to precipitate [37]. Thus, to study adsorption behavior, the pH of the solution of <6 was selected. Figure 2 presents the Cu 2+ adsorption performance by the modified-MCN biochars with various pH values of the solution for one hour with the adsorbent dose of 0.3 g/L. The figure clearly shows that the adsorption capabilities of Cu 2+ by MCN biochars modified with K2CO3, H3PO4 and H2O2 increases along with an increase in the pH of the solution. The Cu 2+ removal percentages significantly increased from the pH of the solution of 2.5 to 4. Further, an increase in pH from 4-5.5 witnesses a negligible increase in adsorption efficiencies. For the K2CO3-modified MCN biochar, the removal percentages of Cu 2+ at pH values of 4, 4.5, 5 and 5.5 were 22.66%, 28.27%, 32.61% and 33.85%, respectively, indicating that the optimal pH for Cu 2+ removal by the K2CO3-modified MCN biochar was 5-5.5. When Cu 2+ was absorbed by H3PO4-modified MCN biochar, a similar trend in the effect of pH on the adsorption performance was also observed and the maximum adsorption of ions was 55% obtained at a pH of 5-5.5. Therefore, a pH of the solution of 5.5 was optimal for the maximum Cu 2+ adsorption of 76%. These results are consistent with that of previous studies [38,39]. It is relevant to note that the MCN biochar modified by H2O2 showed the highest Cu 2+ removal in comparison with that modified by K2CO3 and H3PO4.

Adsorption of Cu 2+ by Modified-MCN Biochar
It is well-known that the pH of the solution plays a significant role in the adsorption behavior of the Cu 2+ , which is related to the dissolution and precipitation of copper [36]. With a pH of less than 6, copper is mostly present in the aqueous solution as ions, however, when the pH of the solution >6, copper ions tend to precipitate [37]. Thus, to study adsorption behavior, the pH of the solution of <6 was selected. Figure 2 presents the Cu 2+ adsorption performance by the modified-MCN biochars with various pH values of the solution for one hour with the adsorbent dose of 0.3 g/L. The figure clearly shows that the adsorption capabilities of Cu 2+ by MCN biochars modified with K 2 CO 3 , H 3 PO 4 and H 2 O 2 increases along with an increase in the pH of the solution. The Cu 2+ removal percentages significantly increased from the pH of the solution of 2.5 to 4. Further, an increase in pH from 4-5.5 witnesses a negligible increase in adsorption efficiencies. For the K 2 CO 3 -modified MCN biochar, the removal percentages of Cu 2+ at pH values of 4, 4.5, 5 and 5.5 were 22.66%, 28.27%, 32.61% and 33.85%, respectively, indicating that the optimal pH for Cu 2+ removal by the K 2 CO 3 -modified MCN biochar was 5-5.5. When Cu 2+ was absorbed by H 3 PO 4 -modified MCN biochar, a similar trend in the effect of pH on the adsorption performance was also observed and the maximum adsorption of ions was 55% obtained at a pH of 5-5.5. Therefore, a pH of the solution of 5.5 was optimal for the maximum Cu 2+ adsorption of 76%. These results are consistent with that of previous studies [38,39]. It is relevant to  The concentrations of the adsorbents had significant impacts on the Cu 2+ and Zn 2+ adsorption performances of the activated biochar. Figure 3 shows the Cu 2+ adsorption efficiency of chemically modified MCN biochars with a pH of the solution of 5 and an adsorption time of 1 h. The adsorption capacities of MCN biochars activated with K2CO3, H2O2 and H3PO4 increased along with adsorbent concentrations. For the K2CO3-activated MCN biochar, the Cu 2+ adsorption efficiency increased remarkably with dosed of 0.2-1.4 g/L. It gradually increased with dosage before reaching the maximum of 84.96% of Cu 2+ removal at the adsorbent concentration of 2 g/L. Similar trends could also be observed with H2O2 and H3PO4-activated MCN biochars, where the Cu 2+ adsorption efficiencies increase with an increase of adsorbent concentrations and reaching a maximum of 80.50% and 94.53% at a concentration of 2 g/L H2O2 and H3PO4-activated MCN biochars, respectively. With the dose of 2 g/L, the MCN biochars modified with H3PO4 exhibited the highest Cu 2+ removal efficiency in comparison with that modified with H2O2 and K2CO3.  The concentrations of the adsorbents had significant impacts on the Cu 2+ and Zn 2+ adsorption performances of the activated biochar. Figure  The concentrations of the adsorbents had significant impacts on the Cu 2+ and Zn 2+ adsorption performances of the activated biochar. Figure 3 shows the Cu 2+ adsorption efficiency of chemically modified MCN biochars with a pH of the solution of 5 and an adsorption time of 1 h. The adsorption capacities of MCN biochars activated with K2CO3, H2O2 and H3PO4 increased along with adsorbent concentrations. For the K2CO3-activated MCN biochar, the Cu 2+ adsorption efficiency increased remarkably with dosed of 0.2-1.4 g/L. It gradually increased with dosage before reaching the maximum of 84.96% of Cu 2+ removal at the adsorbent concentration of 2 g/L. Similar trends could also be observed with H2O2 and H3PO4-activated MCN biochars, where the Cu 2+ adsorption efficiencies increase with an increase of adsorbent concentrations and reaching a maximum of 80.50% and 94.53% at a concentration of 2 g/L H2O2 and H3PO4-activated MCN biochars, respectively. With the dose of 2 g/L, the MCN biochars modified with H3PO4 exhibited the highest Cu 2+ removal efficiency in comparison with that modified with H2O2 and K2CO3.   Figure 4 shows the effect of adsorption time on the Cu 2+ removal efficiencies by the modified MCN biochars with an adsorbent content of 2 g/L, Cu 2+ concentration of 30 ppm and solution pH of 5. It can be clearly seen that the optimal time to adsorb Cu 2+ by the MCN biochars activated with K 2 CO 3 was 30 min with an efficiency of 84.02%, which became saturated at 40 min of processing time at a removal efficiency of 86.35%, after which the treatment efficiency increased insignificantly at 50 min to 87.85% and slightly decreased at 1 h to 87.81%. Research results determined that pH = 5, a dosage of 2 g/L, and a processing time of 30 min was optimal for treating Cu 2+ . Thus, it shows that K 2 CO 3 -activated MCN biochar could be used effectively as an adsorbent for the treatment of toxic Cu 2+ in textile wastewater. Badruddoza et al. (2011) [33] found that that after 30 min of treatment, the processing efficiency of Cu 2+ using carboxymethyl-cyclodextrin conjugated magnetic nanoparticles had a similar treatment performance of 90% removal. Research results from Singha and Das (2013) [40] showed that after 5 h of treatment, the efficiency of Cu 2+ treatment at pH 6 using activated carbon from coconut shell was approximately 90%.
5. It can be clearly seen that the optimal time to adsorb Cu 2+ by the MCN biochars activated with K2CO3 was 30 min with an efficiency of 84.02%, which became saturated at 40 min of processing time at a removal efficiency of 86.35%, after which the treatment efficiency increased insignificantly at 50 min to 87.85% and slightly decreased at 1 h to 87.81%. Research results determined that pH = 5, a dosage of 2 g/L, and a processing time of 30 min was optimal for treating Cu 2+ . Thus, it shows that K2CO3-activated MCN biochar could be used effectively as an adsorbent for the treatment of toxic Cu 2+ in textile wastewater. Badruddoza et al. (2011) [33] found that that after 30 min of treatment, the processing efficiency of Cu 2+ using carboxymethyl-cyclodextrin conjugated magnetic nanoparticles had a similar treatment performance of 90% removal. Research results from Singha and Das (2013) [40] showed that after 5 h of treatment, the efficiency of Cu 2+ treatment at pH 6 using activated carbon from coconut shell was approximately 90%.
For the MCN biochar modified with H3PO4 the Cu 2+ removal efficiencies with reaction times of 0, 10, 20, 30, 40, 50 min and one hour were determined to be 0, 85.08%, 92.58%, 93.21%, 95.92%, 96.12%, 96.14% and 96.03%, respectively. This indicates that the H3PO4-activated MCN biochar has a quick absorbing capability for Cu 2+ with the highest efficiency of 96.14% after 50 min of adsorption time. When activated with H2O2, the MCN biochar also reveals fast removal of Cu 2+ as 51.58% is removed only after 10 min. The optimized adsorption time for Cu 2+ treatment using H2O2-activated MCN biochar is one hour with the highest Cu 2+ removal efficiency of 80.9%. It can be concluded that the MCN biochar activated with H3PO4 and K2CO3 show faster Cu 2+ adsorption efficiencies than that activated with the H2O2 agent.

Adsorption of Zn 2+ by Modified-MCN Biochar
The pH of the solution had a significant effect on Zn 2+ adsorption performance of biochar. Thus, the effect of pH of the solution on the removal efficiency of Zn 2+ by the modified-MCN biochar for one hour with the adsorbent dose of 0.3 g/L was investigated as shown in Figure 5. In general, the adsorption capabilities of Zn 2+ by MCN biochars modified with K2CO3, H3PO4, and H2O2 decreased at a low pH of the solution of 2 to 3, reached a minimum value at a pH 2.5-3, then significantly increased in a pH of 3.5-5. The adsorption of Zn 2+ by MCN biochars activated with K2CO3 decreased with the increase in the pH of the solution from 2 to 3 and reached a minimal removal concentration of 2.63 ppm at a pH of 3. Further increases in the pH of the solution demonstrated an increase in the adsorption capabilities of biochar; it reached a maximum at the pH of 4.5 with a removal concentration of 4.85 ppm. A similar trend was also observed with the adsorption behaviors of the H2O2-modified biochar with the change in the pH of the solution from 2 to 5. A Zn 2+ removal concentration of 2.18 ppm was achieved at a pH of 3 and a maximum of 6.27 ppm at a pH of 4.5.

Adsorption of Zn 2+ by Modified-MCN Biochar
The pH of the solution had a significant effect on Zn 2+ adsorption performance of biochar. Thus, the effect of pH of the solution on the removal efficiency of Zn 2+ by the modified-MCN biochar for one hour with the adsorbent dose of 0.3 g/L was investigated as shown in Figure 5. In general, the adsorption capabilities of Zn 2+ by MCN biochars modified with K 2 CO 3 , H 3 PO 4 , and H 2 O 2 decreased at a low pH of the solution of 2 to 3, reached a minimum value at a pH 2.5-3, then significantly increased in a pH of 3.5-5. The adsorption of Zn 2+ by MCN biochars activated with K 2 CO 3 decreased with the increase in the pH of the solution from 2 to 3 and reached a minimal removal concentration of 2.63 ppm at a pH of Sustainability 2020, 12, 7909 7 of 11 3. Further increases in the pH of the solution demonstrated an increase in the adsorption capabilities of biochar; it reached a maximum at the pH of 4.5 with a removal concentration of 4.85 ppm. A similar trend was also observed with the adsorption behaviors of the H 2 O 2 -modified biochar with the change in the pH of the solution from 2 to 5. A Zn 2+ removal concentration of 2.18 ppm was achieved at a pH of 3 and a maximum of 6.27 ppm at a pH of 4.5. Interestingly, for the H 3 PO 4 -activated macadamia biochar, the lowest Zn 2+ removal concentration was observed to be 1.33 ppm at the pH of 2.5 and the highest removal efficiency was at the pH of 4.5 with a removal concentration of 6.05 ppm. These results indicate that the most suitable pH solution for the removal of Zn 2+ from the aqueous solution by activated macadamia biochar was around 4.5 and the H 2 O 2 -activated biochar reveals the highest Zn 2+ adsorption capability. Interestingly, for the H3PO4-activated macadamia biochar, the lowest Zn 2+ removal concentration was observed to be 1.33 ppm at the pH of 2.5 and the highest removal efficiency was at the pH of 4.5 with a removal concentration of 6.05 ppm. These results indicate that the most suitable pH solution for the removal of Zn 2+ from the aqueous solution by activated macadamia biochar was around 4.5 and the H2O2-activated biochar reveals the highest Zn 2+ adsorption capability.  Figure 6 shows the effect adsorbent concentrations of the chemically modified MCN biochars on the removal performance of Zn 2+ from the aqueous solution at a pH of 4.5 for one hour. The Zn 2+ removal concentrations increased with the increase in adsorbent dosed. For the K2CO3-activated biochar as adsorbent, Zn 2+ removal increased significantly in the adsorbent doses ranging from 0.2 g/L to 2 g/L and reaches a maximum at the adsorbent dose of 2 g/L with the highest removal percentage of 45.80%. However, compared to the removal efficiency of 1.8 g/L (45.29%), this value was not significant, and thus, the optimal K2CO3-activated biochar concentration for cost-effective adsorption of Zn 2+ adsorption was determined to be 1.8 g/L. This trend was also observed with H2O2 and H3PO4-activated MCN biochars as the optimal adsorbent doses were determined to be 1.8 g/L for Zn 2+ adsorption efficiencies of 57.3% and 65.56%, respectively. Figure 7 shows the effect of adsorption time (0 min to 120 min) on the Zn 2+ removal efficiencies of the chemically modified MCN biochars with the adsorbent content of 1.8 g/L, Zn 2+ concentration of 30 ppm, and a pH of 4.5. Unlike the removal of Cu 2+ , for which the chemically activated macadamia biochars showed high adsorption speed reaching the equilibrium state only after 30 min, in this case, the adsorption ability of the biochars for Zn 2+ oxides were relatively slow. The Zn 2+ adsorption efficiencies of K2CO3, H2O2 and H3PO4-modified macadamia biochars only reached the equilibrium value after 80 min of adsorption time with the removal concentrations of 12.22 ppm, 15.48 ppm and 16.42 ppm, respectively.
The results discussed above lead to the conclusion that the macadamia biochars activated with H3PO4 show the highest and fastest Zn 2+ and Cu 2+ adsorption performances, and therefore, can be employed as effective adsorbents for the removal of metal ions from aqueous media.  Figure 6 shows the effect adsorbent concentrations of the chemically modified MCN biochars on the removal performance of Zn 2+ from the aqueous solution at a pH of 4.5 for one hour. The Zn 2+ removal concentrations increased with the increase in adsorbent dosed. For the K 2 CO 3 -activated biochar as adsorbent, Zn 2+ removal increased significantly in the adsorbent doses ranging from 0.2 g/L to 2 g/L and reaches a maximum at the adsorbent dose of 2 g/L with the highest removal percentage of 45.80%. However, compared to the removal efficiency of 1.8 g/L (45.29%), this value was not significant, and thus, the optimal K 2 CO 3 -activated biochar concentration for cost-effective adsorption of Zn 2+ adsorption was determined to be 1.8 g/L. This trend was also observed with H 2 O 2 and H 3 PO 4 -activated MCN biochars as the optimal adsorbent doses were determined to be 1.8 g/L for Zn 2+ adsorption efficiencies of 57.3% and 65.56%, respectively. Figure 7 shows the effect of adsorption time (0 min to 120 min) on the Zn 2+ removal efficiencies of the chemically modified MCN biochars with the adsorbent content of 1.8 g/L, Zn 2+ concentration of 30 ppm, and a pH of 4.5. Unlike the removal of Cu 2+ , for which the chemically activated macadamia biochars showed high adsorption speed reaching the equilibrium state only after 30 min, in this case, the adsorption ability of the biochars for Zn 2+ oxides were relatively slow. The Zn 2+ adsorption efficiencies of K 2 CO 3   It is clear from the above results that the MCN biochars activated with H3PO4 reveal the highest removal efficiency toward Cu 2+ and Zn 2+ . The adsorption capacity can be roughly estimated with the following equation: where C0 (mg/L) is the initial concentration, Ce (mg/L) is the equilibrium concentration, V (L) is the solution volume and m (g) is the mass of the activated MCN biochars. Based on the investigation of adsorbent dosage on the removal efficiency toward Cu 2+ and Zn 2+ with the adsorption dosage of 0.2 g/L, Cu 2+ and Zn 2+ concentration of 30 mg/L, the adsorption capacity of activated MCN biochars for Cu 2+ and Zn 2+ are 2.825 and 2.1 mg/g, respectively. These results are slightly higher than the adsorption capacity of the biochar fabricated from the rice husk for the Cu 2+ and Zn 2+ removal [41].   It is clear from the above results that the MCN biochars activated with H3PO4 reveal the highest removal efficiency toward Cu 2+ and Zn 2+ . The adsorption capacity can be roughly estimated with the following equation: where C0 (mg/L) is the initial concentration, Ce (mg/L) is the equilibrium concentration, V (L) is the solution volume and m (g) is the mass of the activated MCN biochars. Based on the investigation of adsorbent dosage on the removal efficiency toward Cu 2+ and Zn 2+ with the adsorption dosage of 0.2 g/L, Cu 2+ and Zn 2+ concentration of 30 mg/L, the adsorption capacity of activated MCN biochars for Cu 2+ and Zn 2+ are 2.825 and 2.1 mg/g, respectively. These results are slightly higher than the adsorption capacity of the biochar fabricated from the rice husk for the Cu 2+ and Zn 2+ removal [41]. The results discussed above lead to the conclusion that the macadamia biochars activated with H 3 PO 4 show the highest and fastest Zn 2+ and Cu 2+ adsorption performances, and therefore, can be employed as effective adsorbents for the removal of metal ions from aqueous media.
It is clear from the above results that the MCN biochars activated with H 3 PO 4 reveal the highest removal efficiency toward Cu 2+ and Zn 2+ . The adsorption capacity can be roughly estimated with the following equation: where C 0 (mg/L) is the initial concentration, C e (mg/L) is the equilibrium concentration, V (L) is the solution volume and m (g) is the mass of the activated MCN biochars. Based on the investigation of adsorbent dosage on the removal efficiency toward Cu 2+ and Zn 2+ with the adsorption dosage of 0.2 g/L, Cu 2+ and Zn 2+ concentration of 30 mg/L, the adsorption capacity of activated MCN biochars for Cu 2+ and Zn 2+ are 2.825 and 2.1 mg/g, respectively. These results are slightly higher than the adsorption capacity of the biochar fabricated from the rice husk for the Cu 2+ and Zn 2+ removal [41].

Conclusions
To summarize, the macadamia biochar was successfully fabricated and modified with K 2 CO 3 , H 2 O 2 and H 3 PO 4 . The modified-MCN biochars have porous microstructures with an average pore size of 10 µm. The resultant chemically modified biochars were used as adsorbent and they show high Cu 2+ and Zn 2+ adsorption performances. The effect of several factors such as the pH of the solution, adsorption time and adsorbent doses were investigated in detail. The results showed that the optimized pH of the solution, adsorbent doses and adsorption time for the removal for Cu 2+ and Zn 2+ are 5, 2 g/L and 30 min and 4.5, 1.8 g/L and 80 min, respectively. Of the three K 2 CO 3 , H 2 O 2 and H 3 PO 4 modifiers, the macadamia biochars activated with H 3 PO 4 had the highest Cu 2+ and Zn 2+ adsorption performances. With high adsorption efficiencies and inexpensive fabrication from biomass waste, chemically activated macadamia biochar can be used as a promising adsorbent for the effective removal of toxic metal ions in practical applications.