While zirconium-metal-organic frameworks (Zr-MOFs) show promise for arsenic remediation, their practical implementation faces critical challenges: poor capture of neutral arsenite (H₃AsO₃ containing As(III)) and limited selectivity for arsenate (HAsO₄²⁻ and H₂AsO₄⁻ containing As(V)) species in the presence of a host of competing ions. Here, we present a transformative solution using dimercaptosuccinic acid (DMSA) ligands to construct two topologically distinct Zr-MOFs. The Zr-DMSA framework with fcu topology demonstrates exceptional capture of both arsenic species, even in the presence of phosphates, which normally compete for adsorption sites greatly decreasing performance. Through comprehensive structural analysis combining pair distribution function analysis, X-ray absorption spectroscopy, and solid-state NMR, we uncover the molecular basis for this superior performance. Comparative studies with analogous thiol-free Zr-MOFs reveal that DMSA's thiol groups enable strong covalent sulfur-arsenic interactions. Moreover, DFT calculations and XAS analysis illuminate an unexpected mechanism; framework defects, namely dangling DMSA ligands in the MOF pores, enhance As(III) chelation. This finding challenges the conventional wisdom that high crystallinity is essential for MOF performance, suggesting instead that controlled defect engineering can dramatically enhance functional properties. The remarkable selectivity for arsenate over phosphate likely stems from arsenate's reduction potential, enabling its conversion to As(III) during thiol chelation. This work not only addresses a pressing environmental challenge but establishes defect engineering as a powerful strategy for enhancing MOF performance in selective guest capture.