University of Saskatchewan
Department of Soil Science
College of Agriculture and Bioresources

Environmental Soil Chemistry

ATR-FTIR spectroscopy of boric Acid adsorption on hydrous ferric oxide (HFO)


Boron is an essential micronutrient for plant growth. Like many micronutrients, boron is required in low concentrations for the growth of plants but can be quite toxic to plants at higher levels. The range between these extremes is quite narrow. Borate toxicity is a major concern for agriculture in both arid regions of the United States and in poorly drained soils of high salinity. High levels of borate are also commonly found in irrigation water. Borate deficiency has also been reported in over forty U.S. states. Boric acid B(OH)3 and its anion borate B(OH)4- have solution chemistry that is quite different from most other oxyanions. Borate forms by the addition of a hydroxyl group to the trigonal planar boric acid molecule, forming a tetrahedral anion. The pK of this reaction is ~ 9.3.


Hydrous ferric oxide is a good proxy for amorphous iron hydroxide coatings that are important geosorbents in natural systems. Boric acid has a high affinity for both amorphous and crystalline iron hydroxides.

Experimental:

A Perkin Elmer 1720X FTIR spectrometer was used for all spectroscopic studies. The instrument was equipped with a Whatman purge gas generator to remove water vapor and CO2, a N2(l)-cooled MCT detector, and a horizontal ATR-FTIR accessory from Spectra-Tech. A deposition method was employed for adsorption studies using a Spectra Tech ATR-flow cell with a 45 degree ZnSe crystal. A more detailed explanation of the deposition method can be found in Peak et al. 1999. The primary advantage of the deposition technique is that it results in an extremely high solid concentration at the ZnSe crystal surface so the concentration of surface species in the path of the infrared beam is much higher than when applying pastes or slurries of mineral sorption samples. As a result, bulk solution concentrations of reactant can remain well below infrared detection limits while adsorption reactions proceed. The spectra collected are therefore only surface complexes of reactant, and typically have sharper and more easily resolved peaks than other ATR-FTIR techniques.


To make peak assignment simpler, it is possible to consider the O-H groups of boric acid and borate a single entity. This allows one to approximate the molecular symmetry of boric acid using a trigonal planar BX3 molecule and of borate as a tetrahedral BX4. The symmetry for these types of molecules has been fully worked out in Nakamoto (1986). One can then predict how the number and position of infrared peaks is expected to change when boric acid and borate form bonds with Fe3+ in HFO: .

Figure 1.Theoretical splitting of boric acid and borate infrared bands as bonds form with Fe(III). Note is that it is not possible to assign monodentate vs. bidentate bonding for trigonal boric acid complexes using FTIR.

Results:

At pH 6.5, FTIR spectra showed that both trigonal and tetrahedral boron surface complexes form with HFO. As three peaks are present in the 1250-1400 cm-1 range, there must be more than one trigonal complex present. Although there is virtually no borate in solution at pH 6.5, tetrahedral boron surface complexes are still seen on HFO. At pH 10.4, much more tetrahedral boron is present. However, a substantial amount of trigonal boron is still present at the HFO surface. One of the peaks (at 1395 cm-1) from the pH 6.5 spectra are absent at pH 10.4.

Figure 2. ATR-FTIR spectra of boric acid/borate adsorbed on HFO at pH 6.5 and pH 10.4. Fits of infrared peaks are grouped into different surface complexes below the total spectra.


When the results are all integrated, it is possible to propose probable mechanisms of boric acid and borate adsorption onto HFO. Initially, boric acid interacts with HFO by formation of an outer-sphere intermediate that is more properly called a Lewis acid-base pair (analogous to ion pairs in solution). Next, a reaction between boric acid and surface functional groups can result in either a trigonal or a tetrahedral boron surface complex. Solution pH will affect the complex that forms due to the leaving groups in the ligand exchange. At neutral pH, the trigonal surface complex is preferred because the leaving group is H2O. At high pH, the neutralization of the H+ leaving group by OH- in solution makes tetrahedral surface complex formation more favorable. The rapid decline in adsorption above boric acid's pKa is due to the reactive boric acid species being converted to borate in solution. At extremely high pH, boric acid could directly bond with an Fe-O- group. It is not likely for borate to react with Fe-OH groups, as a five-coordinate intermediate would be required.

Figure 3.Proposed mechanism for boric acid bonding with the HFO surface based upon ATR-FTIR spectroscopy.


Important Findings



References


1) Nakamoto, K. (1986) Infrared and Raman Spectra of Inorganic and Coordination Compounds. John Wiley and Sons, New York.
2) Peak, D., R. G. Ford, and Sparks, D.L. (1999) An in situ ATR-FTR investigation of sulfate bonding mechanisms on goethite. J. Colloid Interface Sci. 218, 289-299.


Publications:
Peak, D., G.W. Luther III, and D.L. Sparks. 2003 "Boric acid and borate adsorption mechanisms on amorphous iron oxides: An in situ ATR-FTIR spectroscopic study." Geochimica et Cosmochimica Acta 67(14): 2551-2560.