Neem tree (Azadirachta indica) leaves were collected from twigs of a number of matured tall neem trees within and near the main campus of Umaru Musa Yar’adua University, Katsina. The samples were thoroughly washed with tap-water, rinsed copiously with distilled water to remove dust and any other soluble substances. The leaves were allowed to air dry under shade at room temperature until they become crisp. The dried leaves sam- ples were then pulverized with a mechanical grinder into a powdered; and then dried overnight for 16 hours in an oven at a temperature of 65˚C. The oven-dried neem-tree leaves powder (NLP) samples were sieved to the working sizes of 75 - 300 μm using electronic sieve shaker and the fractions preserved in separately labelled air-tight plastic containers according to their particle sizes. Similar procedure was carried out on sliced pieces of fresh watermelon (Citrullus lanatus) rinds samples, collected from local fruit vendors at Kofar Kaura and Cen- tral Market in Katsina Metropolis, with the powdered fractions (WRP) separately preserved in plastic containers [21] [22] .

The analytical grade Congo red dye supplied by BDH Laboratory was used as received. Stock solution of the dye was prepared by dissolving 1 g solute in 1000 cm³ volumetric flasks to make 1000 mg/L of the dye solution [23] . Model and working calibration standards were prepared by serial dilution of the stock so- lution.

Experiments on the adsorption of Congo red by the adsorbents (WRP and NLP) were carried out by batch me- thod and the influence of various parameters such as contact time (5 - 240 min), adsorbent dosage (100 - 500 mg), particle size (, , , and), initial dye concentration (5 - 300 mg/L) and initial dye pH (2 - 12) were studied at constant agitation speed of 300 rpm and room temperature (25˚C) in triplicates. The adsorption measurements were conducted by mixing various amounts of adsorbent in 150 cm³ Erlenmeyer glass flasks containing 50 cm³ of dye solution of known concentration. The initial pH of the dye solutions were adjusted to the desired values by adding few drops of 0.1 M HCl or 0.1 M NaOH aqueous solutions. The solutions were agitated using orbital shaker for a predetermined time to attain equilibrium after which, the samples were removed and the supernatant solution was separated from the adsorbent by filtration using Whatman No. 41 filter paper, discarding the first few volume (3 - 4 drops) of the filtrate [24] . The filtrates were used for analyses using UV-visible spectrophotometer, reporting each data point as an average value of the triplicates recorded. In each case, the percentage adsorption and substrate’s equilibrium adsorption capacity, (mg/g) were evaluated using Equations (1) and (2) respectively.

where (mg/L) is the initial dye concentration, is the concentration at equilibrium or predetermined time t, (L) is the volume of dye solution used and (g) is the weight of the adsorbent. The data obtained were tested against the linear forms of Langmuir, Freundlich, Temkin, Dubinin-Radushkevich (D-R) and Harkins- Jura isotherms, respectively represented as;

where C_e is any liquid phase concentration of the dye in equilibrium with the adsorbent, q_e is equilibrium ad- sorption capacity of the adsorbent, q_m is monolayer capacity, q_D (mg/g) is the theoretical monolayer saturation capacity of adsorbent, K_L is Langmuir adsorption constant, K_F is Freundlich constant for relative adsorption ca- pacity of adsorbent, A_T is the Temkin isotherm equilibrium binding constant (L/g), B_T is the Temkins heat of adsorption, A_HJ is Harkins-Jura isotherm parameter which accounts for multilayer adsorption and explains the existence of heterogeneous pore distribution, while is the isotherm constants [24] .

Based on the relationship of adsorption capacity for CR dye adsorption onto the adsorbents and the equilibrium concentrations, the Langmuir adsorption isotherms are modeled and presented in Figure 1. According to these

isotherm curves, the Langmuir isotherm parameters are calculated and listed in Table 1.

As shown in Table 1, CR adsorption on both adsorbents have the same value of linear regression coefficient, 𝑅² (0.9998), suggesting that the experimental data agreed closely with each other. However, the negative values of R_L and K_L indicates unfavourable adsorption of the dye onto the adsorbents [25] .

Based on the relationship of adsorption capacity for CR dye adsorption onto the adsorbents and the equilibrium concentrations, the Freundlich adsorption isotherms are correlated and given in Figure 2, while the isotherm parameters are as presented in Table 1.

From the Table 1, CR adsorption on the adsorbents have a range of values of linear regression coefficient,R² (0.9913 - 0.9935), demonstrating that the experimental data fitted well with the Freundlich isotherm equation, third to the Langmuir isotherm. Moreover, it was reported that the Freundlich isotherm constant can be used to explore the favourability of adsorption process. The adsorption process is said to be favourable when the value of satisfies the condition, otherwise it is unfavourable. While the values in Table 1 for ad- sorption of CR on watermelon are situated outside the range of 1 - 10 indicating unfavourable adsorption proc- ess, the values for CR adsorption on neem leaves are within the range of 1 - 10, demonstrating favourable ad- sorption process [26] [27] .

Figure 3 illustrates the Temkin isotherm model for the dye adsorption onto the adsorbents from which the rele- vant isotherm parameters are listed in Table 1. It can be discovered in Table 1 that the values of R² are posi- tioned within 0.9932 - 0.9947, which gave a close fit to the CR adsorption on watermelon rind and neem leaves samples, values next only to Langmuir’s model. This outcome suggests that the experimental data fitted better with the Temkin isotherm model [28] . Furthermore, it can also be discovered in Table 1 that the adsorption heat of CR adsorption on watermelon rind and neem leaves samples was restricted within −0.92 to 0.74 kJ/mol.

Making the linear plot according to adsorption capacity for CR dye adsorption onto the adsorbents and the equi- librium concentrations, the Dubinin-Radushkevich (D-R) adsorption isotherms (Figure 4) was obtained. Corre- sponding to which, the isotherm parameters were calculated as in Table 1.

The values of linear regression coefficient (R²) are in the range of 0.9359 - 0.9574, revealing that the experi- mental data fitted well with the Dubinin-Radushkevich (D-R) isotherm model. Moreover, it is reported that when the value of E is below 8 kJ/mol, the adsorption process can be considered as the physical adsorption. In contrast, if the value of E is located in the range of 8 - 16 kJ/mol, it is the chemical adsorption. From Table 1, it can be observed that the obtained values of mean free energy, E, are limited within the range of 0.29 - 0.32

kJ/mol. Based on these data, it can thus be concluded that the effect of physical adsorption will play a dominat- ing role in the adsorption process of CR dye adsorption onto the adsorbents [25] [29] .

The Harkins-Jura isotherm models for CR adsorption onto watermelon rinds and neem-tree leaves samples are presented in Figure 5 and the relevant isotherm parameters (Table 1) shows that the values of R² are located in the range of 0.9889 - 0.9908, which indicate a better fits to the CR adsorption onto watermelon rinds and neem-

tree leaves samples. This result reveals that CR adsorption onto watermelon rinds and neem-tree leaves samples is in support of the multilayer adsorption rule [30] [31] .