Copper-glyphosate sorption to microcrystalline gibbsite in the presence of soluble Keggin Al13 polymers

Among the most reactive yet largely neglected adsorbents of toxicant species occurring in acidic aquatic environments are the is an element of-Keggin Al-13 polyoxocations [AlO4Al12(OH)(24)(H2O)(12)(7+)], known generally as Al-13 polymers. Here, we report on the sorption of Cu(II), a common ingredient of pesticides, and glyphosate {N-[phosphonomethyl]glycine (PMG)}, a widely applied herbicide, to microcrystalline gibbsite [gamma-Al-(OH)(3)] in the presence of soluble Al-13 polymers over the pH range 4-7. In the presence of gibbsite and soluble Al-13 polymers, dissolved Cu(II) decreased gradually with pH, achieving a minimum at pH 5.5. Between pH 5.5 and 6.0, however, soluble Cu increased markedly, with approximately 80% of the added metal remaining in solution at pH 5.86. At pH > 6.0, soluble Cu once again decreased, becoming undetectable at pH 7. The anomalous Cu solubilization was attributed to a concomitant deprotonation of soluble Al-13 polymers, yielding surface OH groups possessing high affinity for Cu(II). Removal of Cu from solution at pH > 6.0 is facilitated by flocculation of the Al-13 polymers to which Cu had sorbed. The sorption behavior of the zwitterionic PMG in the presence of gibbsite and Al-13 polymers was consistent with this interpretation, there being a dramatic increase in sorbed PMG at pH > 6.0 as the Al-13 polymers deprotonated and flocculated. Copper and PMG loss from solution with increasing pH when both adsorptives were added to the gibbsite-Al-13 polymer system was broadly similar to what was observed in the PMG-free systems, although small differences were detected in response to varying the order of adsorptive addition. The inclusion of soluble Al polymers in our experiments exposes a fundamental limitation of models based on but a single inorganic adsorbent as a means to predict the behavior of trace metals and xenobiotic organic compounds in natural systems.