## 4 Conclusions  
The biomass-derived adsorbent was successfully prepared by nitric acid impregnation and showed a high tendency for adsorption of heavy metals with a capacity of 51, 50, 51 and 40 mg g\({}^{-1}\) for Cr(III), Fe(III), Co(II) and Ni(II), respectively. In addition, the prepared adsorbent exhibited a superior efficiency for dispersive solid-phase extraction, exhibiting.LODs of 1.4, 2, 4.1, 1.9 and 3.0 ug.L\({}^{-1}\) for Cr(III), Fe(III), Co(II) and Ni(II), respectively. The LOQs were reported as 4.1, 7.3, 5.7 and 8.9 ug.L\({}^{-1}\) for Cr(III), Fe(III), Co(II) and Ni(II), respectively. In addition, the optimized dispersive solid-phase extraction process exhibited RSD% of 4.3, 4.1, 3.2 and 4.9 for Cr(III), Fe(III), Co(II) and Ni(II), respectively. The proposed extraction procedures have a preconcentration factor of 50. In addition, the extraction method using biomass-derived adsorbent showed a high tolerance ability for various ions including Al\({}^{+3}\), K\({}^{+}\)/Cl\({}^{-}\), Pb\({}^{+2}\), PO\({}_{4}\)\({}^{-3}\), Cd\({}^{+2}\), Mg\({}^{+2}\), Ca\({}^{+2}\), Cu\({}^{+2}\), NO\({}_{3}\)\({}^{-}\), F\({}^{-}\), CO\({}_{3}\)\({}^{-2}\), SO4\({}^{-2}\) and Na\({}^{+}\), indicating the accuracy of the developed dispersive solid-phase extraction procedures.  
The following are available online at [[https://www.mdpi.com/article/10.3390/app11177792/s1](https://www.mdpi.com/article/10.3390/app11177792/s1)]([https://www.mdpi.com/article/10.3390/app11177792/s1](https://www.mdpi.com/article/10.3390/app11177792/s1)), Table S1: ICP-MS operating conditions.  
Data curation, N.A.; formal analysis, T.S.A., N.A. and M.S. ([PERSON]); funding acquisition, N.A.; investigation, M.A.H., Z.A.A. and M.S. ([PERSON]); methodology, T.S.A., M.A.H. and M.S. ([PERSON]); project administration, M.A.H., Z.A.A. and M.S. ([PERSON]); resources, N.A. and Z.A.A.; supervision, Z.A.A.; validation, M.A.H.; writing--original draft, T.S.A.; writing--review and editing, M.A.H. and M.S. ([PERSON]). All authors have read and agreed to the published version of the manuscript.  
This work was funded by the Deanship of Scientific Research at Princess Nourah Bint Abdulrahman University through the research group program grant number RGP-1440-0023.  
Not applicable.  
The data presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy.  
The authors acknowledge the Deanship of Scientific Research at Princess Nourah Bint Abdulrahman University for funding this research through the research group program grant number RGP-1440-0023.  
The authors declare no conflict of interest.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multicolumn{1}{c}{**Extraction Process**} & \multicolumn{6}{c}{**Limits of Detection (ug.L\({}^{-1}\))**} \\ \cline{2-6} Solid-phase extraction of metal ions from fuel ethanol with a nanostructured adsorbent & - & 0.2 & - & 0.46 & [44] \\ \hline Dispersive solid-phase extraction (USE-AA-D-SPE) & & & & & \\ Layered double hydroxides with & & & & & \\
4-amino-5-hydroxyl-2,7-naphthalenedisulfonic acid & 1.7 & - & 2.1 & 2.4 & [13] \\ monosodium salt interlayer anions & & & & & \\ \hline Dispersive micro solid-phase extraction (DMSPE) onto & 0.06 & 0.21 & - & - & [18] \\ graphene oxide (GO) nanosheets & & & & & \\ \hline Magnetic solid-phase extraction using carbon-coated Fe3O4 & 0.002 & - & 0.001 & - & [17] \\ nanoparticles & & & & & \\ \hline Solid-phase extraction using multiwalled carbon nanotubes & & & & & \\ impregnated with 4-(2-thiazol)resercinol & - & - & - & 4.3 & [45] \\ \hline Dispersive solid-phase extraction onto biomass-derived & & & & & \\ adsorbent & & & & & \\ \hline \hline \end{tabular}
\end{table}
Table 5: Comparison of extraction of Cr(III), Fe(III), Co(II) and Ni(II) using various methods from literature.