Assessing 50 Years of Salt Effects in Heterogeneous Photocatalysis

Abstract

Heterogeneous photocatalysis has been studied for the treatment of recalcitrant compounds for half a century, but one of the severe limitations for practical oxidation treatment continues to be the inhibitory effect of salt found in real water matrices. The negative impact of Na+, K+, Cl–, and other ionic species is rarely quantified, as there has been no systematic way to discuss salt effects, making the evaluation of photocatalysis performance across studies difficult and imprecise. In analyzing 50 years of literature for this review, we identified >1,200 publications on heterogeneous photocatalysis in different water types, such as seawater, brackish water, and industrial brines. Just 84 studies reported degradation rates in deionized and salt-containing water, with nearly all involving TiO2 as the photocatalyst composition and charged dye compounds as the model contaminant class. To analyze the kinetic data extracted from these studies, we introduce the photocatalysis salt-effect (PSE) metric as calculated through the relative salt-effectiveness evaluation (RSEE) method, quantifying the percent change in degradation rate when the water matrix contains a salt species. We identified an unexpected correlation between PSE and ionic strength (IS), irrespective of the contaminant, at solution pH values less than the TiO2 point-of-zero-charge (PZC), which is the plausible result of salt-induced particle aggregation at high ionic strengths during photocatalyst testing. While most published results showed salt inhibition of photocatalysis (−100% < PSE < −5%), there is evidence for salt promotion (PSE > +5%) and salt tolerance (−5% < PSE < +5%), which challenges the notion that salinity is always detrimental to photocatalytic activity. These findings highlight the opportunity to study fully and systematically the relationships among contaminant structure, photocatalyst PZC, solution pH, IS, and salt type. The PSE-based framework would be critical in understanding the effects of more complex water matrices and in ultimately developing salt-tolerant and salt-promoted heterogeneous photocatalysis for practical water treatment.