Potentials of mono- and multi-metal ion removal from water with cotton stalks and date palm stone residuals

In this work, cotton stalks (Gossypium barbadense) and date palm stones (Phoenix dactylifera) have been used as biosorbents to remove cadmium; Cd(II), lead; Pb(II), and zinc; Zn(II) from mono- and multi-solutions. Each biosorbent was characterized using SEM-EDX, and FT-IR. The findings showed that pH, dose, contact time, metal concentration, and particle size affect the treatment process. The adsorption pattern was Pb(II) > Cd(II) > Zn(II) for both biosorbents. The adsorption performance of cotton stalks was higher than that of date palm stones. The fitted maximum uptake capacities; qm of cotton stalks were higher than those of date palm stones. The maximum adsorption at optimum conditions of Pb(II), Cd(II), and Zn(II) with cotton stalks were 98%, 92.1%, and 78.9%, respectively, within 30 min. While the maximum adsorption of Pb(II), Cd(II), and Zn(II) with date palm stones were 94.6%, 76%, and 68.6%, respectively. Results confirmed the antagonistic effect of heavy metal removal at optimum conditions. Biosorbents could remove ~ 100% of the metal ions from real wastewater samples. Regeneration investigation revealed a successful reusability of both biosorbents for four cycles. Supplementary information The online version contains supplementary material available at 10.1007/s11356-023-27137-4.

where, qe the quantity of solute adsorbed per unit weight of biosorbent (mg g -1 ).C0 and Ce: are the initial and final metal ions concentration (mg L -1 ), respectively.V and W: are the solution volume (L) and the biosorbent weight (g), respectively.R: is the metal ions removal efficiency (%).
Different models describe the equilibrium between the adsorbate and biosorbent, among these models Langmuir and Freundlich isotherms.The Langmuir biosorption isotherm is often used to describe the maximum biosorption capacity of biosorbent.It is given by eq.6, whereas Freundlich isotherm is expressed by eq. 7.Moreover, the separation factor (SF) is calculated to indicate the feasibility of the adsorption process (eq.8).
Ce/qe= 1/ KLqm + Ce/qm , (S6) where, qm: is the maximum biosorption uptake under the given condition (mg g -1 ).KL: is the coefficient related to the affinity between the sorbent and sorbate (L mg -1 ).Kf: is the Freundlich adsorption constant, which characterizes the strength of adsorption [(mg g -1 ) (mg L -1 ) n ]. n: is the measure of adsorption intensity.SF: is the separation factor (dimensionless).

Regeneration of biosorbents
To conduct the regeneration investigation of the studied biosorbents, 10 g L -1 of each biosorbent was added into Erlenmeyer flasks containing 100 mL of 5, 10, and 15 mg L -1 of Cd(II), Pb(II), and Zn(II), respectively.After equilibration for 30 min under room temperature, the adsorbent was recovered, and the adsorption capacity determined.By washing the used biosorbent surface three times in a row with distilled water, followed by a 60 °C oven dry to constant mass, residual metal ions were removed.
Using 0.2 M HCl, the metal ions were desorbed.The recovered adsorbent was mixed in 30 mL of the desorption agent for 30 min at 150 rpm.The adsorbate-adsorbent mixtures were filtered using Whatman no 1 filter paper.The concentrations of the Cd(II), Pb(II) and Zn(II) ions were determined using Atomic Absorption Spectrometer and the desorption percentage was computed using eq. 1.
Desorption efficiency = (qdes / qads) × 100 , where qdes: is the amount of metal left on the biomass after the desorption process.qads: is metal ion uptake capacity.
The recovered biosorbents were dried at 60 °C, and reused in further adsorption-desorption cycles to determine the reusability of the biomass.The reusability of the biomass was conducted in four successive adsorption-desorption cycles.

Table S1 . FTIR Characterization of the investigated date palm and cotton stalk stems before and after biosorption of Cd (II), Pb (II) and Zn (II) ions
TableS2Biosorption capacity/efficiency (mg g -1 / %) of heavy metals by different biosorbents.