Advanced oxidation of orange G using phosphonic acid stabilised zerovalent iron

Orange G (OG) is an important chemical for the textile industry but also a major water contaminant. It is also a model compound representing more toxic azo dyes and degradation products. Activation of persulphate (PS) by zerovalent iron nanoparticles (nZVI) has been proposed as an effective technique to degrade OG even at neutral pH conditions. However, particle agglomeration leads to diminished reactivity and remains a crucial challenge for nZVI applications. This work evaluates the performance of three different nZVI that are each stabilised with one of three phosphonic acids, ATMP, DTPMP or HTPMP. While ATMP- and DTPMP-stabilised nZVI were practically inert to OG solution in the absence of PS, HTPMP-stabilised nZVI achieved ∼15% total carbon (TC) reduction under similar conditions. However, in the presence of PS, ATMP- and DTPMP-stabilised nZVI resulted in degradation rate constants that were double the rate constant obtained for HTPMP-stabilised nZVI under similar conditions. These results highlight the important role of both stabilisers and PS in modulating nZVI activity with potential niche application. A desirable feature of the PS-nZVI degradation mechanism is the rapid destruction of both the azo bond and benzene ring in OG, resulting in fewer, and less toxic, degradation products than have been previously reported. Significant mineralisation was recorded in longer term experiments, where 90% organic carbon reduction was achieved in 48 h for the three stabilised nZVI. Hence, phosphonate-stabilised nZVI activation of PS is an effective means to achieve complete OG degradation and significant mineralisation with non-toxic major degradation product.