CEPHAPIRIN ADSORPTION IN SAND-IRON SYSTEMS: EFFECTS OF HEMATITE, MAGNETITE, AND ZERO-VALENT IRON

The study of trace amounts of antibiotics in surface and ground water is an emerging field in environmental science, hydrogeology and applied analytical chemistry. One source of contamination is agricultural waste, particularly from dairy operations. Because surface water and ground water are a connected resource, contamination of subsurface aquifers is highly probable. Understanding the fate and transport of antibiotics in the vadose zone and ground water requires the same type of fundamental studies that were applied to petroleum contamination during the 1980's and 90's. One type of fundamental information is experimental data on the mobility of antibiotic compounds in various porous geologic materials.

This presentation reports the results of batch experiments on the adsorption of cephapirin (a widely-used cephalosporin veterinary antibiotic) on various natural sand-iron mixtures. Two grain size fractions of quartz-rich sand were investigated, 0.35-.43 mm diameter (med. sand) and 0.08-.13 mm diameter (v.-fine sand), containing 0.32 wt.% and 0.48 wt% soil organic matter, respectively. Each fraction was mixed with separate amounts of iron filings (Fe^o), magnetite (Fe₃O₄), or hematite (Fe₂O₃) of the same diameter in weight ratios ranging from 99.5% sand: 0.5% iron phase, to 95% sand: 5% iron phase. Experiments were conducted at 21^oC and 39^oC. Aqueous solutions were buffered to a pH of approximately 5 and equilibrium cephapirin concentrations were determined by LC/MS.

Linear adsorption distribution coefficients (in L/kg) for medium sand at 21^oC are from 2.4 (+/- 1.6) to 11.6 (+/- 0.11) in Fe^o mixtures, 2.3 (+/- .06) to 2.5 (+/- .08) in Fe₃O₄ mixtures, and 2.0 (+/- .07 ) to 2.6 (+/- .01) in Fe₂O₃ mixtures. Distribution coefficients (in L/kg) for very fine sand at 21^oC are from 2.1 (+/- .05) to 6.0 (+/- .14) in Fe^o mixtures, 1.3 (+/- 0.26) to 1.4 (+/- 0.18) in Fe₃O₄ mixtures, and 1.4 (+/- 0.13) to 2.0 (+/- 0.11) in Fe₂O₃ mixtures. Higher temperature experiments allowed the calculation of changes in standard thermodynamic properties for the adsorption process. Preliminary data indicate that the addition of small amounts (0.5 to 5% by wt.) of iron phase to sand increases the K_d for cephapirin by a factor of 9. Data also indicate that a reduced form of iron has a larger effect than more oxidized forms.