Kinetics and isotherm analysis of 2,4-dichlorophenoxyl acetic acid adsorption onto soil components under oxic and anoxic conditions

Abstract

2,4-dichlorophenoxyl acetic acid (2,4-D, pKa = 2.8) is used extensively as a herbicide in agricultural practices. Its sorption behavior on both untreated and soils treated to significantly remove specific components (organic and iron and manganese [Fe-Mn] oxides and hydroxides phases) was investigated under oxic and anoxic conditions. The chemical and structural heterogeneity of the soil components were characterized by elemental analysis and X-ray diffraction (XRD). The coexistence of the various components seems to either mask sorption sites on the untreated soil surfaces or inhibit interlayer diffusion of 2,4-D. All sorption data conform to the Freundlich description and a pseudo-second-order kinetic model. There was a strong positive correlation between sorption capacity K(d), and surface area (r(2) ≤ 0.704), but a negative correlation was uncovered with both pH and organic carbon (r(2) ≤ -0.860). The results indicate that 2,4-D is preferably sorbed under oxic rather than anoxic conditions and it is greater on soils containing a high Fe content. There was incomplete 2,4-D sorption reversibility, with desorption occurring more rapidly under anoxic conditions. The study suggests that stimulation of Fe III reduction could be used for the bioremediation of a 2,4-D-contaminated site.

Interactive influence of Fe–Mn and organic matter on pentachlorophenol sorption under oxic and anoxic conditions

“…The treatment not only enhanced the d-spacing from 3.432 in UNTD to 3.581 , 3.553 and 3.494 in S -Mn , S -FeMn and S -OM respectively but also the peaks of isolated samples, indicating that phase transformation occurred on the untreated samples. Similar observations on all the characteristic features of the original and isolated soils in this study were reported in our recent studies [17][18][19][20].…”

“…In addition to the original untreated soil (UNTD), soil samples were also prepared with three different reagents to extract target components. Details have been published in our recent report on sorption of glyphosate, 2,4,6-TCB, 2,4-D and PFOS [17][18][19][20]. Briefly, 0.1 M NH 2 OHÁHCl and 0.01 M HNO 3 were used for 30 min to remove manganese oxides, and the product is denoted as S -Mn [36].…”

“…The cation exchange capacity (CEC) was determined following the silver thiourea method [41]. The specific surface area, organic carbon contents, total extractable and pseudo-total amounts of Fe and Mn in the samples were determined following standard procedures as reported recently [17][18][19][20]. The crystallinity of the samples was determined by X-ray diffraction, XRD (MD10) with Cu Ka radiation at 1.5406 Å.…”

“…Thus, contrary to previous report on the predominant nature of organic material on sorption of hydrophobic organic compounds (HOC) [15][16][17], the study further justified the significance of the mineral oxide phases, particularly the role of Fe oxide in the sorption of PCP. Recent studies on sorption of 2,4,6-trichlorobenzoic acid (2,4,6-TCB), 2,4-dichlorophenoxyl acetic acid (2,4-D) and perfluorooctane sulphonate (PFOS) onto soil components revealed the significance of mineral phases in sorption of HOCs [18][19][20]. Some studies have shown that at typical environmental pH values (pH [7][8], several chlorophenols, including PCP are usually present in both molecular and phenolate form.…”

“…The chemically mediated oxidation-reduction potential may alter the chemical and physical characteristics of the soil including local pH increases, leading to increased dissolution of soil organic matter. Studies have shown that sorption is governed by competing sorbates as dissolved organic matter (DOM) in ligand exchange, as well as pH and ionic strength [18][19][20]32,33]. In addition, as redox potential decreases further, soils/sediments release manganese Mn 2+ and iron Fe 2+ via the microbiological and chemical reduction of metal oxides [18,19,34].…”