The oxidative absorption of hydrogen sulfide (H2S) into a solution of ferric chelate of trans-1,2-diaminocyclohexanetetraacetate (CDTA) was studied in a counter-current laboratory column randomly packed with 15 mm plastic Ralu rings. The present investigation takes concern about the Kraft pulping situation where dilute H2S concentrations are omnipresent in large-volume gas effluents. A fractional two-level factorial approach was instigated to determine the significance of six operating variables, namely the solution's alkalinity (pH; 8.5-10.5), the liquid mass flow rate (L; 1.73-5.19 kg m(-2) s(-1)), the solution's ionic strength (I-C; 0.01-0.1 mol dm(-3)), the gas mass flow rate (G; 0.19-0.57 kg m(-2) s(-1)), the inlet H2S concentration (C-H2S,C-0; 70-430ppm) and the initial ferric CDTA concentration (C-Fe,C-0; 100 -400 mu mol dm(-3)). Initially, a Plackett-Burman design matrix of seven duplicated experiments revealed that pH is the leading factor controlling the H2S conversion rate while the ionic strength and ferric CDTA concentration effects remained negligible within the factorial domain. Surface response analysis based on 11 duplicated factorial experiments plus 10 central composite trials revealed that the H2S conversion significantly increases with liquid flow rate but decreases with growing H2S load up. Further examination about the influence of ferric CDTA on H2S absorption rate was set up over a broader concentration range (C-Fe,C-0; 0-2000 mu mol dm(-3)) at pH of 9.5 and 10.5. It showed good potential at 2000 mu mol dm(-3) as H2S conversion increased by a significant 25% for both pH values in comparison to pure alkaline solutions containing no ferric CDTA. (c) 2005 Elsevier Ltd. All rights reserved.