Phosphate interference in molybdenum blue-based arsenic analysis: arsenate-phosphate mimicry and its impact on environmental monitoring and water treatment evaluation

Abstract

The reliable quantification of arsenic in water is essential for environ-mental monitoring, regulatory compliance, and the evaluation ofremediation technologies. Among available methods, the molybde-num blue (MoBlue) assay remains widely used due to its simplicity,low cost, and adaptability to low-resource settings. However, itscontinued application to arsenate determination raises critical con-cerns that extend beyond routine analytical limitations. This CriticalReview shows that arsenate quantification by the MoBlue method isfundamentally constrained by arsenate-phosphate chemical mimi-cry. Because both oxyanions form structurally and spectroscopicallyindistinguishable heteropolymolybdate complexes, the resultingoptical signal reflects total heteropoly blue formation rather thanthe identity of the central anion. Consequently, no modification ofreaction conditions can provide true molecular selectivity underenvironmentally relevant phosphate-containing conditions. This lim-itation has far-reaching implications for environmental research. Instudies of adsorption materials, photocatalysts, membranes, andengineered treatment systems, MoBlue signals are frequently inter-preted as direct measures of arsenic removal. In realistic aqueousmatrices, however, the response may be dominated by phosphate ormixed contributions, leading to systematic errors in reported arsenicconcentrations, removal efficiencies, adsorption capacities, andkinetic parameters. By critically analysing recent literature and pro-posed mitigation strategies, this review highlights a systematic ana-lytical limitation that is frequently overlooked in MoBlue-basedarsenate quantification workflows and discusses its implications forthe evaluation of arsenic remediation technologies. Furthermore, thisanalytical process was supported by the supplementary use of artifi-cial intelligence tools. The goal of this review is not to quantitativelyre-evaluate previously published remediation efficiencies, but rather to emphasise the fundamental selectivity limitations inherent toMoBlue-based arsenate determination in phosphate-containing sys-tems. It further advocates a transition towards element-specificdetection methods to ensure reliable arsenic quantification androbust assessment of water treatment performance in complex envir-onmental matrices.