Photochemical reduction of Cr(VI) coupled with Ni(II) adsorption by thiourea-functionalized magnetic coconut shell carbon: Experimental and DFT insights

Nowadays, heavy metals are a major issue worldwide, polluting natural resources. In the given study, a novel approach was developed; commercial coconut shell activated carbon was modified with Fe 3 O 4 and thiourea (CH 4 N 2 S) to adsorb Cr(VI) and Ni(II) from industrial wastewater under photochemical reaction on a signal platform. The Cr(VI) reduction-adsorption rate was approximately 95% (2.91 mg/L), which was investigated under acidic conditions (pH = 2) with a 40 mg dose of adsorbent. The same phenomenon was observed for Ni(II) adsorption, approximately 91% (3.98 mg/L) at pH = 6 for 60 min, using the same adsorbent dose. The increased removal efficiency is attributed to the synergetic behavior of Fe₃O₄ and thiourea, where the Fe₃O₄ endorsed the reduction-adsorption of Cr(VI) while thiourea functional groups ( NH₂ and C S) endorsed the adsorption of Ni(II) with strong coordination of active sites along with surface complexation and electrostatic interactions. Analysis indicated the breakage of the activated carbon structure after the Cr(VI) adsorption. To get insights into the adsorption mechanism, Density Functional Theory calculations were performed to optimize structure, showing the stability of metal and surface complexes. FMO analysis demonstrates a reduction in the HOMO-LUMO energy gap from 2.95 eV (CSAC) to 0.17 eV (CSAC@Fe₃O₄@TU & metal), indicating increased surface reactivity and charge transfer. While NCI and RDG calculations further confirm the electrostatic attraction and bonding of Cr(VI) and Ni(II). This study presents a novel approach to conducting comparative studies under photochemical reactions. The CSAC@Fe 3 O 4 @Tu is a low-cost, green catalyst that was used for the remediation of environmental pollutants.