Display omitted •CHI/PEC porous scaffolds have been employed as the host matrix to immobilize microporous MOFs.•The composite membrane fully retains the record AsIII and AsV adsorption capacity of the MOF.•There is a synergic function to photo-oxidize AsIII to AsV when assembling MOF and CHI/PEC in a composite membrane.•The multi-step mechanism for the dual AsIII to AsV oxidation and adsorption has been detailed. The presence of arsenic as neutral arsenite and oxoanionic arsenate species are a worldwide issue that requires an effective remediation technology capable to remove both AsIII and AsV from water. In this research we propose the assembly of a membrane build up from chitosan/pectin (CHIPEC) composite biopolymer and the UiO-66-NH2 porous and photo-active metal–organic framework. CHIPEC@UiO-66-NH2 membrane integrates porosity, adsorptive capacity, and activity to photo-oxidize and adsorb AsIII and AsV species. Nanometric-sized UiO-66-NH2 was synthetized and homogeneously distributed within the CHIPEC matrix, achieving a composite membrane with optical physico-chemical and optical properties (band gap of 2.91 eV) suitable to be applied on photooxidation of AsIII and adsorption of AsV. Under optimal conditions, maximum adsorption capacities (Qmax) values of 254 and 168 mg g−1 were reached for AsIII adsorption by MOF and membrane, respectively. Notably, for AsV adsorption, Qmax values of 268 and 335 mg g−1 were reached for powdered and immobilized MOF, respectively. Moreover, the membrane keeps most of its activity for AsV adsorption when employed in surface and groundwater matrixes. Further, the efficiently of UiO-66-NH2 to photo-oxidize AsIII to AsV, subsequently adsorbing the arsenate oxyanions, is improved by a factor of five when is integrated into the polymer. A detailed multi-step mechanism for the arsenic transformation and adsorption in CHIPEC@UiO-66-NH2 has been proposed on the basis of the anionic exchange at Zr hexa-nuclear clusters and amino groups in the MOF. Overall, MOF-biopolymeric composite has been proved as a straightforward, efficient, low-cost, and environmentally friendly solution for detoxifying arsenic in surface and underground water.