The decrease in plant extract's acidity leads to the increase in Lewis acid site densities of greenly synthesized magnetite surfaces. The increase in Lewis acid site densities leads to the increase in azo dye adsorption yields.

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Magnetite NP samples synthesized from less acidic plant extracts have more dense active sites and prefer adsorbing more MG. The increase of plant extract acidity leads to the increase of the thermo- and photocatalytic activities of Fe3O4 NPs.

The aim of this paper is to study the effect of Evans blue (EB) and methyl orange (MO) functional group contents on the photocatalytic activities of four magnetite nanoparticle surfaces under UV-Vis (365 nm) in ambient conditions against the photodegradation of EB and MO. For this purpose, the adsorption has been conducted, at first, in dark ambient conditions to eliminate the effect of adsorption. The residual solutions of dye/magnetite were then used for photocatalytic reactions of the free functional groups of MO and EB on the four magnetite surfaces. The novelty of this paper lies in the demonstration of the effect of EB and MO functional groups on photocatalytic activities of magnetite nanoparticles (NPs) through the electron-hole pair’s interactions with dyes functional groups translated by the photoactivation of the Br ̈onsted and Lewis acid sites on the magnetite NPs. The results found show that MO photodegradation capacities and rates are higher than those of EB on the four magnetite surfaces, this is probably due to a large number of chromophores and auxochromes present in MO initial residual solutions. This is related to the effect of chromophore and auxochrome contents on the photoactivation of Br ̈onsted and Lewis acid sites. High chromophore and auxochrome contents in MO residual solutions further promote the photoactivation of Br ̈onsted and Lewis acid sites, resulting in the greater formation of trapped-electron and trapped-hole sites. The latter will then photoreduce more chromophores and photooxidize more auxochromes, respectively, which leads to an increase in the photocatalytic activity of magnetite surfaces.

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