Iron oxide-mediated hydrolysis of phthalate and organophosphate esters under indoor-relevant temperature and humidity: Kinetics, molecular structure effects, and acidic product accumulation.

Abstract

Ester-containing semi-volatile organic compounds (SVOCs), including phthalates and organophosphate esters, are ubiquitous indoor pollutants, yet their transformation on reactive indoor surfaces remains poorly understood. Here, we investigated the hydrolysis of representative phthalate and organophosphate esters on hematite, used as a model iron oxide surface, under indoor-relevant temperature and relative humidity (RH) conditions. Controlled hydrolysis experiments showed that these compounds underwent hematite-mediated hydrolysis, with apparent kinetics governed by RH, temperature, and molecular structure. Increasing RH markedly suppressed apparent hydrolysis of alkyl-substituted esters, whereas phenyl-containing esters exhibited faster hydrolysis and weaker RH dependence. Molecular dynamics simulations of interfacial partitioning supported a water-film accessibility mechanism, whereby thicker water films restricted ester-surface contact and inhibited hydrolysis. For phenyl-containing esters, the weaker RH dependence was associated with structure-dependent chemical and interfacial effects, including the greater leaving-group ability of phenoxy groups and compound-specific surface accessibility. Accelerated gas-phase exposure experiments further showed that dimethyl phthalate supplied through gas-phase adsorption could undergo hydrolysis on hematite and chemically complex iron-rich surfaces, including aged corrosion materials. These findings identify hematite and iron oxide-rich surfaces as reactive indoor interfaces that may transform phthalate and organophosphate ester SVOCs, promote acidic product accumulation, and potentially alter their surface retention and indoor fate. Because the apparent kinetics were determined under a nominal surface loading selected for reliable quantification, they are best interpreted as RH-, temperature-, and structure-dependent trends under the present experimental conditions, warranting future quantification under realistic indoor surface loadings and broader surface conditions.