## 3 Results and Discussion
### Development of the Dispersive Solid-Phase Extraction Using the Biomass-Derived Adsorbent  
The adsorption capacity of the prepared biomass-derived adsorbent towards uptake of Cr(III), Fe(III), Co(II) and Ni(II) from the aqueous solution of 25 mg.L\({}^{-1}\) was investigated (Figure 5A). The increasing adsorbent mass led to a decrease in the adsorption capacity, with a maximum adsorption capacity of 51, 50, 51 and 40 mg g\({}^{-1}\) for Cr(III), Fe(III), Co(II) and Ni(II), respectively, using an adsorbent mass of 0.01 g.L\({}^{-1}\). The further increase in adsorbent mass from 0.04 to 0.1 g.L\({}^{-1}\) did not show a noticeable change due to the fixation of the metal ion solution concentration (25 mg.L\({}^{-1}\)) [37]. The adsorbent mass of 0.01 g was selected as the optimal mass in all extraction procedures. The preconcentration experiments were performed using the dispersive solid-phase extraction techniques for enrichment of Cr(III), Fe(III), Co(II) and Ni(II). The developed dispersive solid-phase extraction was optimized by studying the effect of shaking time on the recovery of heavy metals (Figure 5B), indicating that 10 min is enough to achieve a quantitative recovery percentage. The developed biomass-derived adsorbent is rich with surface carbonyl and hydroxyl groups, which enhance the fast uptake of heavy metals.  
The developed dispersive solid-phase extraction was further optimized by evaluating the effect of pH of the extraction medium and the possible elution by acids, as well as the ability to tolerate the coexisting ions [38; 39]. The effect of pH on the recovery percentage of Cr(III), Fe(III), Co(II) and Ni(II) was studied in the range of 2 to 8; pH 4 exhibited the highest recovery percentage, as presented in Figure 6A. In neutral and basic media, the  
Figure 4: FTIR spectrum of the original palm waste (**A**) and biomass-derived adsorbent (**B**).  
Figure 5: Adsorption capacity in relation to mass of adsorbent (**A**) and effect of shaking time on the recovery percentage of Cr(III), Fe(III), Co(II) and Ni(II) during dispersive solid-phase extraction onto biomass-derived adsorbent (**B**).  
recovery percentage decreased, which may be attributed to the weak interaction between heavy metal ions and the biomass in presence of a solution rich in hydroxyl groups.  
The starting sample volume of the metal solution including Cr(III), Fe(III), Co(II) and Ni(II) was changed from 15 to 70 mL during the extraction process, and the related recovery percentage for Cr(III), Fe(III), Co(II) and Ni(II) is presented in Figure 6B. The recovery percentage for Cr(III), Fe(III), Co(II) and Ni(II) was found to be quantitative up to 50 mL. Under constant dose of heavy metal content in the sample, the increase in sample volume led to two effects: the first effect is that the overall metal concentration is diluted in the sample, and the second effect is that the metal ions have to migrate for longer distance to be adsorbed on the biomass solid phase, leading to low recovery percentage.  
The efficiency of the eluent acid was studied by reporting the recovery percentage of Cr(III), Fe(III), Co(II) and Ni(II) using 1 mL of 0.5 M nitric acid, 1 M nitric acid, 0.5 M acetic acid, 1 M acetic acid, 2 M acetic acid and 3 M acetic acid (Figure 6C). The elution with 1 mL of 0.5 M nitric acid was the most suitable elution of all the tested metals including Cr(III), Fe(III), Co(II) and Ni(II). The lower recovery using acetic acid as eluent is due to the weak acidity, which exhibited weak elution power for the adsorbed Cr(III), Fe(III), Co(II) and Ni(II), while nitric acid is known as a strong acid and has high dissolving power for elution.