*2.3.3 Zeta potential*

Zeta potential of banana peel biomass and rice straw were measured using a Zetasizer Nano-ZS (Malvern Instruments, UK).

### **2.4 Solutions of toxic metal ions**

Solutions were prepared to be used in the isotherm, pH and kinetics tests. For each metal (Cu (II) and Pb (II)), a 1000 mg L−1 solution was used as the standard solution for other solutions of concentration: 0.5, 5, 20, 50, 100 and 200 mg.L−1. The contaminating reagents used were Dehydrated Copper (II) Chloride - Sigma 99% and Lead (II) Nitrate - Cinética.

#### **2.5 Adsorption tests**

#### *2.5.1 Isotherm test*

A quantity of 10 mg of the adsorbent were used in 10 ml samples of the metal solutions at concentrations of 0, 0.5, 5, 20, 50, 100 and 200 mg.L−1. Afterwards, they were placed in a Thermo Scientific shaker model 4360 for 24 hours. After 24 hours, the samples were taken to a CIENTEC CT-6000R centrifuge for 3 minutes at 5500 rpm (4058 g) and the aliquot removed to dilute to 2% nitric acid. The initial and residual concentrations were analyzed by NexION 300D PerkinElmer (USA) ICP-MS and the adsorbed amount (qe) was calculated by Eq. (1):

$$q\_{\epsilon} = \frac{V\left(C\_{o} - C\_{\epsilon}\right)}{m} \tag{1}$$

where Co and Ce (mg.L−1) are the initial and equilibrium concentrations respectively, V is the volume of the solution (L) and m is the mass (g) of the adsorbent used in the experiments.

The Langmuir model (Eq. (2)) is used for a monolayer adsorption process on a homogeneous surface, in which the concentration occurs at specific sites in the adsorbent. The equation of the Langmuir isotherm is given by Eq. (2):

$$q\_{\epsilon} = \frac{K\_L Q\_{\text{on}} \mathbf{C}\_{\epsilon}}{\mathbf{1} + K\_L \mathbf{C}\_{\epsilon}} \tag{2}$$

where Qm (mg.g−1) and KL (L.mg−1) are the maximum adsorption capacity and the Langmuir constant used for adsorption energy, respectively.

The adsorption is favorable (0 < RL < 1), unfavorable (RL > 1), linear (RL = 1) or irreversible (RL = 0) (Eq. (3)) [11].

$$R\_L = \frac{1}{1 + K\_L C\_o} \tag{3}$$

The Freundlich model (Eq. (4)) is used to describe multilayer adsorption on heterogeneous surfaces.

$$\mathbf{q}\_e = \mathbf{K}\_F \mathbf{C}\_e^{1/n} \tag{4}$$

where KF and 1/n are the Freundlich constants representing adsorption capacity and adsorption intensity, respectively.
