Mechanisms for removal of p-nitrophenol from aqueous solution using zero-valent iron

Batch experiments were conducted to examine mechanisms for removal of p-nitrophenol (PNP) from aqueous solution using zero-valent iron (ZVI). Removal of PNP using ZVI was mainly attributed to three mechanisms: degradation, precipitation and adsorption. A complete removal of 30 mg L(-1) PNP with ZVI dosage of 1000 mg L(-1) achieved within 30 min at pH 3. The PNP removal rate in the acidic solutions was significantly suppressed at higher pH. The modified Langmuir-Hinshelwood kinetic model could successfully describe the PNP removal process using ZVI at different pH conditions. Total organic carbon (TOC) removal efficiencies were found to be almost independent of pH. While the TOC removal at lower pH was profoundly affected by the reductive and/or oxidative degradation, the adsorption was favorable at higher pH. The effect of dissolved oxygen on PNP removal was investigated at pH 3 where a maximum contribution of oxidative degradation could be expected. The PNP removal in the anoxic system purged with nitrogen gas was quick as well as that in the system being open to the air. However, the TOC removal under the anoxic condition was negligible as compared with that in the oxic system. The profiles of the intermediates formed during the PNP degradation indicated that the reductive degradation was predominant in the initial phase of the removal and subsequently the oxidative degradation occurred.