The decoloration of reactive azo dye, Reactive Black 5 (RB5), by an advanced Fenton process in conjunction with ultrasound (Fenton/US) was investigated. The Fenton/US process using zero-valent iron (ZVI) aggregates as catalyst could achieve synergistic degradation of RB5, as compared to Fe(0)/US and Fenton systems. A synergy factor of 22.9 based on the first-order rate constant (k) was found. The decoloration efficiency was strongly influenced by initial pH, ZVI dose, H2O2 dose, ultrasonic input power, and the presence of salts in reaction solution. Experimental results showed that the optimum conditions for the decoloration of RB5 were ZVI 1 g/L, initial pH 3.0, and H2O2 1.03 x 10(-2) mol/L with acoustic power of 120 W/L at 60 kHz. These conditions yielded 99% decoloration of 5.0 x 10(-4) M RB5 (ADMI 5605) solution within 10 min treatment and the operation cost was only 2.25 USD/m(3). The dye solutions containing inorganic anions could retard the decoloration. The inhibitory effect of various inorganic anions on decoloration of RB5 followed the sequence of H2PO4- >> Cl- ClO4- approximate to NO3-SO42-. High acoustic power accelerated the reaction rate and increased decoloration efficiency. ZVI aggregates were reusable and the decoloration efficiency did not decrease substantially with repeated use of ZVI. Findings showed that the Fenton/US process could effectively decolor the reactive azo dye RB5 in wastewater. (C) 2013 Elsevier B.V. All rights reserved.