Removal of 2,4-Dichlorophenoxyacetic acid from water and organic by-product minimization by catalytic ozonation

Abstract

Background: 2,4-dichlorophenoxyacetic acid (2,4-DCPA acid) is a toxic herbicide. Earlier studies to remove 2,4-DCPA acid from water used expensive and/or toxic reagents, resulting in the formation of toxic organic by-products (Org-BPs). This study evaluates the removal of 2,4-DCPA acid from aqueous media using uncatalysed and catalytic ozonation with Fe doped with Ni and Co respectively.

Methods: Mixed metal oxides of Ni and Co loaded on Fe respectively, prepared by co-precipitation and physical mixing were used as catalyst for ozone facilitated oxidation degradation of 2,4-DCPA acid. Their surface properties were determined by employing SEM, BET and NH₃-TPD. HPLC, IC and TOC data were used for quantifying substrate and oxidation products.

Results: Conversion of 2,4-DCPA acid increased from 38% in acidic water to 73% in basic water, however, only 26% of the total carbon was removed and 9.5% in the form of Org-BPs. With 7:3 Fe:Ni (Co-ppt) catalyst (surface area 253 m² g^-1; particle size 236 nm), 97% of pollutant was converted. Most importantly, 92% of carbon was removed and Org-BP formation was minimized to 1.5%. With 7:3 Fe:Ni (Mixed) catalyst (surface area 12 m² g^-1; particle size 1274 nm), 68% of 2,4-DCPA acid was converted, while 23% of TOC was removed, however, 66% of Org-BP's still remained.

Conclusion: In uncatalysed ozonation degradation of 2,4-DCPA acid improved with the increase in hydroxide ion concentration. Ozonation in presence of 7:3 Fe:Ni (Co-ppt) catalyst resulted in highest activity for dechlorination, TOC removal and Org-BP minimization, thus improving the quality of contaminated water.

Keywords: 7:3 Fe:Ni (co-ppt); Catalytic ozonation; Organic by-products; Total organic carbon.

Fig. 1

Experimental set-up: 1 – medical grade oxygen gas cylinder, 2 – pressure regulator, 3 – ozonator with flowmeter, 4 – reaction vessel with bubbler, stirrer bar and catalyst material, 5 – stirrer, 6 – excess ozone trap containing 0.2 mol dm⁻³ potassium iodide solution

Fig. 2

Percent conversion of 2,4-DCPA acid (a) and TOC removal (b) from acidic, neutral and basic water as a function of time

Fig. 3

Percent yield of Org BP’s (a) and organic acids (b) in acidic, neutral and basic water as a function of time

Fig. 4

Percent chloride released during ozonation of 2,4-DCPA acid in acidic, neutral and basic water as a function of time

Fig. 5

Percent conversion of 2,4-DCPA acid (a) and TOC removal (b) as a function of ozone treatment time during Fe:Ni (Co-ppt) catalytic ozonation

Fig. 6

SEM micrograms for 9:1 Fe:Ni (Co-ppt) and 9:1 Fe:Ni (Mixed) catalyst material

Fig. 7

Percent yield Org-BP’s (a) and organic acids (b) in the presence of Fe-Ni (Co-ppt) as a function of ozone treatment time

Fig. 8

N₂ adsorption-desorption isotherms of (a) Fe:Ni (Co-ppt) and (b) Fe:Ni (Mixed) catalyst material

Fig. 9

Comparison of percent conversion of 2,4-DCPA acid (a) and TOC removal (b) for uncatalyzed ozonation and ozonation as a function of time, in the presence of 30% Ni loaded on Fe by co-precipitation and physical mixing

Scheme 1

Adsorption of water on Fe-Ni (Co-ppt) catalyst surface showing formation of acidic site

Scheme 2

Adsorption of water on Fe-Ni (Mixed) catalyst surface showing formation of basic site

Fig. 10

NH₃-TPD profile of Fe-Ni (Co-ppt) and Fe-Ni (Mixed) catalyst material

Fig. 11

Comparison of Org-BP’s (a) and chloride yield (a) for uncatalyzed ozonation and ozonation in the presence of 30% Ni loaded on Fe by co-precipitation and a simple mixing method as a function time

Fig. 12

Kinetic curves for 2,4-DCPA removal through ozone aeration (a) effect of pH; and (b) effect of Ni loading on Fe