The oxidation of arsenite (As(III)) via Mn-oxides is an important process for natural arsenic (As) cycling and for developing in situ strategies for remediation of As-contaminated waters. In this study, the influence of goethite (alpha-FeOOH), phosphate, and bacteria/biopolymer coatings on the initial As(III) oxidation kinetics by a hydrous Mn-oxide (delta-MnO(2)) is examined via both batch experiments and rapid scan ATR-spectroscopy. Under natural conditions the presence of various mineral surfaces, bacteria, organic matter, and ions in solution can block Mn-oxide reaction sites, alter reaction rates, and thus inhibit As(III) oxidation. Previous studies of As-Mn systems demonstrate rapid oxidation of As(III), catalyzed by Mn-oxides, producing less toxic and mobile arsenate (As(V)). Subsequent to oxidation, reaction products from reductive dissolution of delta-MnO(2) by As(III), bind to and passivate the mineral surface. This study demonstrates enhanced passivation through interaction with phosphate and bacteria. Increased As oxidation with high concentrations of goethite is observed, attributed to As(V) sorption to alpha-FeOOH and diminished surface passivation of delta-MnO(2). Specific competition between phosphate and As(V) for delta-MnO(2) was confirmed through diminished As sorption and decreased As(V) production when oxidation occurred in the presence of phosphate. Kinetic experiments reveal that the extent of initial As(III) oxidation in the presence of low phosphate and alpha-FeOOH concentration is reduced; however, initial reaction rates are generally not affected. Reaction rates are reduced when bacterial adhesion and high phosphate concentrations strongly passivate delta-MnO(2) and reduce As(III) interactions with the mineral surface. The data presented in this study highlight the importance of considering natural heterogeneity when investigating reaction mechanisms and initial reaction kinetics.