Sustainable development in the removal and photocatalytic reduction of heavy metals in wastewaters using environmentally friendly and health benign Porphyrin-based Metal-Organic frameworks

Porphyrin is a rigid, electron-rich, and highly colored chromophore and with anchoring groups, such as carboxylates, can be an important unit of the scaffolding building porous materials called metal–organic frameworks, MOFs. The metallic nodes are generally polymetallic oxides, and together with porphyrins, form semiconducting materials. The porosity, absorption of visible light, material tailoring to modulate the particle dimension, relative ease for post-functionalization, and facile metalation of the corresponding free base, make these MOFs for multitude applications, notably removal of heavy metals from wastewaters. This review assembles the current advances in the removal of heavy elements and toxic metals, namely Cr(III), Cr(VI), Mn(II), Fe(VI), Co(II), Cu(II), Zn(II), Mo(VI), Pd(II), Ag(I), Cd(II), Sb(III), Pt(IV), Au(III), Hg(II), Pb(II), Bi(III), Rb(I), Sr(II), Cs(I), U(VI) and iodine, as well as the visible light driven heterogeneous photocatalytic reduction of highly harmful metal ions into less hazardous or less soluble metallic cations or nanoparticles (Cr(VI) → Cr(III), U(VI) → U(IV), Pd(II) → Pd(0)) by the porous, highly colored porphyrin-based MOFs in water. The pH-dependent capture of the metal ions are generally driven by five main strategies; 1) ionic interactions or H-bonds with the MOF skeleton, 2) coordination of metal cations by the porphyrin macrocycle free base, 3) post-functionalization of a chelating arm inside the MOFs, 4) photochemical process rendering the metal ion less toxic, and 5) photochemical rendering the metal ion less soluble, thus remaining trapped. This work covers other relevant topics such as structures of the porphyrin-based MOFs, photoinduced electron transfer and formation of reactive oxygen species of types I, comparison of the adsorption capacities with other MOFs, assessment of the no or negligible toxicity of these MOFs, discussion on the scale-up, activation of MOFs and the notable neglected toxic metal ions. This detailed and critical review is the most complete survey on what these multifunctional MOFs can do in this area.