Table 1 lists the BET, surface area, pore volume and pore diameter results for MHP and MHPD adsorbents.
The effect of pH on the removal of hexavalent chromium by 20 MHP, 35 MHP, 50 MHP, 20 MHPD, 35 MHPD, and 50 MHPD was carried out in batch adsorption experiments by varying the pH from 1 to 12 while the initial concentration of Cr(VI) was 150 mg/L and the adsorbent dose was 2.5 g/L.
To investigate the feasibility and efficiency of Cr(VI) adsorption onto MHP and MHPD adsorbents, contact time was varied from 20 to 180 min, while all other parameters were kept constant.
Pseudo-first-order  and pseudo-second-order  kinetic models were employed to study the kinetic parameters governing the adsorption of Cr(VI) to MHP and MHPD adsorbents.
It can be seen in Figures 8(a) and 8(c) that the adsorbents exhibited different mechanisms, that is, linear line over the entire time interval (20 MHP, 35 MHP, 20 MHPD, and 50 MHPD) and multilinearity (50 MHP and 35 MHPD).
The magnitudes of [q.sub.e] varied as follows: 4.9 to 15 mg/g for 20 MHP, 5.1 to 20 mg/g for 35 MHP, 5.1 to 40 mg/g for 50 MHP, 6 to 70 mg/g for 20 MHPD, 7 to 50 mg/g for 35 MHPD, and 7 to 74 mg/g for 50 MHPD.
To account for the interaction between Cr(VI) and MHPD adsorbents, nonlinear Langmuir and Freundlich adsorption isotherms were used to model the adsorption equilibrium data from the effect of concentration.
Figures 10(a) and 10(b) portray the %R of Cr(VI) as a function of adsorbent dosage for MHP and MHPD adsorbents.
The percent removal of Cr(VI) decreased from 80 to 79% for 20 MHP, 87 to 80% for 50 MHP, 95 to 93% for 20 MHPD, and 85 to 82% for 50 MHPD, while the %R was exactly the same for 35 MHPD but the removal was higher in the presence of co-ions when 35 MHP was used.
Figure 12 shows the results of %R of Cr(VI) from the spiked solution by MHPD adsorbents.
Successful grafting was further corroborated by the TGA curves of MHPD showing high volatility due to the presence of DPC compared to MHP.
Abbreviations DPC: 1,5'-diphenylcarbazide MHP: Macadamia nutshell powder oxidized by hydrogen peroxide solutions 20 MHP: Macadamia nutshell powder oxidized by 20% (v/v) hydrogen peroxide solutions 35 MHP: Macadamia nutshell powder oxidized by 35% (v/v) hydrogen peroxide solutions 50 MHP: Macadamia nutshell powder oxidized by 50% (v/v) hydrogen peroxide solutions 20 MHPD: 20 MHP adsorbent grafted with DPC 35 MHPD: 35 MHP adsorbent grafted with DPC 50 MHPD: 50 MHP adsorbent grafted with DPC BET: Brunauer-Emmett-Teller DTA: Differential thermal analysis EDX: Energy dispersive X-ray FTIR: Fourier transform infrared spectroscopy LOD: Limit of detection PFO: Pseudo-first order PSO: Pseudo-second order RMN: Raw Macadamia nutshell SEM: Scanning electron microscopy TGA: Thermogravimetric analysis.