Degradation or Adsorption? Revisiting Organic Dye Removal by Two Morphologies of InVO4

Abstract

Indium vanadate (InVO4) has attracted considerable attention for photocatalytic applications owing to its narrow band gap, non-toxicity, and high chemical stability. However, for the removal of organic dyes from wastewater, both photocatalytic degradation and adsorption by InVO4 have been reported, and the dominant mechanism remains unclear. This study investigates the removal mechanism of cationic dyes by InVO4. Two distinct morphologies of InVO4, spherical (IVO-S) and nanorod (IVO-R), were synthesized via the hydrothermal method. Comparative removal tests under light and dark conditions suggesting that adsorption, rather than photocatalytic degradation, is the dominant removal pathway under the conditions investigated. Among the two morphologies, IVO-R exhibits superior adsorption performance achieving a maximum capacity of 290 mg g-1 for methylene blue (MB). This is attributed to its smaller particle size. The material demonstrates excellent pH tolerance maintaining high adsorption efficiency across a broad pH range (2–10), with an optimal dosage of 0.15 g L-1. Zeta potential analysis confirms a negatively charged surface within this pH range. Adsorption was significant only for cationic dyes suggesting electrostatic attraction as the key driving force. Furthermore, XPS analysis reveals no evidence of strong chemical bonding between MB and the InVO4 surface implying that non-covalent interactions between MB and InVO4 surface sites underpin the adsorption process. Kinetic studies indicate that adsorption follows a pseudo-second order model, with a rate constant (k2) of 0.009 g mg-1 min-1 for 30 mg L-1 MB. Isotherm analysis shows the best fit with the Sips model predicting a maximum adsorption capacity of 285 mg g-1 closely matching experimental results.