## 4 Conclusions  
Activated carbon spent coffee grounds synthesised by chemical activation with KOH was tested in this study to assess its ability to remove CV and Cr (VI) from aqueous solutions. The study of the effects of operating parameters showed that 0.1 g/100 mL of ACSCGs removes 95% of the CV from a solution of initial concentration 100 mg/L at pH 9, and 0.15 g/100 mL of ACSCGs removes 95% of Cr (VI) from the same solution at pH 6. The equilibrium adsorption data can be well described for both pollutants by the Langmuir isotherm and the kinetic data are well-fitted by the pseudo-second order model. In addition, the analysis of the data by the intraparticle diffusion model showed that the process takes place in three steps and that intraparticle diffusion is not the only step that limits the adsorption of the two contaminants. The negative values of Gibbs free energy (\(\Delta\)G\({}^{\circ}\)), enthalpy (\(\Delta\)H\({}^{\circ}\)), and entropy (\(\Delta\)S\({}^{\circ}\)) for both pollutants indicate that the adsorption process is spontaneous and exothermic, and leads to a decrease in disorder at the solid-liquid interfaces. Therefore, the synthesized activated carbon can be effectively used in various adsorption applications such as adsorption of cationic dyes and heavy metals from industrial waste.  
Conceptualization, methodology, resources, supervision, validation, writing--review and editing, H.L., A.A.A., F.B. and P.S.A.; formal analysis, data curation, writing--original draft preparation, I.L., M.J. and A.A.; investigation, writing--review and editing, A.K., C.H. and M.O.; project administration, software, investigation, writing--review and editing, Y.C. All authors have read and agreed to the published version of the manuscript.  
this research was funded by Deanship of Scientific Research at Umm Al-Qura University 23 UQU4331101 DSR01.  
Not applicable.  
Some or all data, models, or code that support the findings of this study are available from the corresponding author upon reasonable request (list items).  
The authors would like to thank the Deanship of Scientific Research at Umm Al-Qura University 23 UQU4331101 DSR01.  
The authors declare no conflict of interest.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**adsorbent** & **Precursor** & **Activation Agent** & **pH** & **qm (mg/g)** & **Reference** \\ \hline \multirow{8}{*}{CV} & Sawdust & H\({}_{3}\)PO\({}_{4}\) & 6.5 & 25.2 & [89] \\  & Pineapple waste & KOH & 5 & 31.24 & [90] \\  & Corn cobs & H\({}_{3}\)PO\({}_{4}\) & 10 & 41.80 & [91] \\  & Pomegranate husks & H\({}_{3}\)PO\({}_{4}\) & 11 & 76.92 & [92] \\  & Apple wood & ZnCl\({}_{2}\) & 8 & 142.85 & [93] \\  & Almond shells & KOH & 10 & 625.0 & [94] \\  & Spent coffee grounds & KOH & 9 & 352.1 & This study \\ \hline \multirow{8}{*}{Cr (VI)} & Pine needles & KOH & 4 & 65.36 & [95] \\  & Sugar cane bagasse & ZnCl\({}_{2}\) & 1 & 277.78 & [96] \\  & Eucalyptus sawdust & H\({}_{3}\)PO\({}_{4}\) & 5.5 & 125 & [97] \\  & Orange peel & ZnCl\({}_{2}\) & 1 & 133.33 & [81] \\  & Peach pits & H\({}_{3}\)PO\({}_{4}\) & 5.6 & 14.045 & [98] \\  & Aloe vera waste & H\({}_{2}\)SO\({}_{4}\) & 1.23 & 58.83 & [83] \\  & Spent coffee grounds & KOH & 6 & 187.6 & This study \\ \hline \hline \end{tabular}
\end{table}
Table 8: Comparison of ACSCGs’s adsorption capacity with other activated carbons.