EDA results revealed the key contribution of orbital interactions to the adsorption stability. We performed electron density-based analyses to address the chemical fingerprint of the surface complexation. We evaluate the electron density values (ρi) at the intermolecular In the case of orbital interactions, some differences arise between trivalent and pentavalent methylarsenicals. Orbital interactions increase for trivalent pollutants depending on the adsorption orientation because arsenic atom must be closer to phosphorous atoms. The As-P bonding is associated with ρi values of 0.06-0.07 e/Bohr 3 in the PhosO-DMA III and PhosO-TMA III complexes as expected for orbital interactions (blue values, Fig. ), in agreement with the high ECT contribution in these cases (see section 3.2). The NBO procedure clarifies the orbital picture of the bonding, where PhosO-TMA III and PhosO-TMA V complexes are considered as representative cases. Accordingly, the P-As covalent bond occurs by overlapping of 3p (phosphorous) and sp 3 hybridized (arsenic) orbitals, forming one As-P covalent bond by sharing one electron pair (Fig. ). The bonding density is 60% polarized to methylarsenicals due to their acceptor character; as a result, the As-P bonding strength is lower compared to standard P-P or As-C bonds. In In the case of pentavalent methylarsenicals, the short orbital O-P interactions (dP-O1.8 Å) show high ρi values of 0.10-0.14 e/Bohr 3 (blue values, Fig. ). The O-P interaction is a coordinative covalent bond due to electron delocalization from 2p oxygen lone pair orbitals in methylarsenicals toward one low-occupied *P-P bond of PhosO (Fig. ). Provided that the acceptor antibonding *P-P bond is occupied by 0.3 electrons, destabilizing steric energy must increase in agreement with higher EPAULI energies in the case of adsorbed As(V) species. The O-P bonding features also allow the balance between stable adsorption without pollutant decomposition and desorption ability for further recovery.