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

The reliable quantification of arsenic in water is essential for environmental monitoring, regulatory compliance, and the evaluation of remediation technologies. Among available methods, the molybdenum blue (MoBlue) assay remains widely used due to its simplicity, low cost, and adaptability to low-resource settings. However, its continued application to arsenate determination raises critical concerns that extend beyond routine analytical limitations. This Critical Review shows that arsenate quantification by the MoBlue method is fundamentally constrained by arsenate-phosphate chemical mimicry. Because both oxyanions form structurally and spectroscopically indistinguishable heteropolymolybdate complexes, the resulting optical signal reflects total heteropoly blue formation rather than the identity of the central anion. Consequently, no modification of reaction conditions can provide true molecular selectivity under environmentally relevant phosphate-containing conditions. This limitation has far-reaching implications for environmental research. In studies of adsorption materials, photocatalysts, membranes, and engineered treatment systems, MoBlue signals are frequently interpreted as direct measures of arsenic removal. In realistic aqueous matrices, however, the response may be dominated by phosphate or mixed contributions, leading to systematic errors in reported arsenic concentrations, removal efficiencies, adsorption capacities, and kinetic parameters. By critically analysing recent literature and proposed mitigation strategies, this review highlights a systematic analytical limitation that is frequently overlooked in MoBlue-based arsenate quantification workflows and discusses its implications for the evaluation of arsenic remediation technologies. Furthermore, this analytical process was supported by the supplementary use of artificial intelligence tools. The goal of this review is not to quantitatively re-evaluate previously published remediation efficiencies, but rather to emphasise the fundamental selectivity limitations inherent to MoBlue-based arsenate determination in phosphate-containing systems. It further advocates a transition towards element-specific detection methods to ensure reliable arsenic quantification and robust assessment of water treatment performance in complex environmental matrices.