Sorption of glyphosate and phosphate by variable-charge tropical soils from Tanzania

Sorption of glyphosate and phosphate by four contrasting soils from Tanzania, an Andisol (Sasanda), two Oxisols (Lubonde, Mlingano) and an Ultisol (Nkundi), with variable-charge clay minerals was studied by batch sorption experiments, during which glyphosate and phosphate were added separately as well as together (competitive sorption). Agreement was found between glyphosate and phosphate sorption and between sorbed glyphosate/phosphate and contents of aluminium and iron extractable by oxalate and dithionite-citrate-bicarbonate (oxides and allophane/imogolite). The Langmuir sorption maxima of glyphosate ranged from 15.5 mmol kg− 1 (Nkundi) to 126 mmol kg− 1 (Sasanda), while that of phosphate varied from 5.8 mmol kg− 1 (Nkundi) to 78.5 mmol kg− 1 (Sasanda). Nearly double the amount of glyphosate than phosphate was sorbed, indicating dominance of monodentate glyphosate-soil surface complexes, while the prevailing phosphate-soil complexes seem to be bidentate. Therefore, phosphate appears more strongly bonded than glyphosate, which agrees with much higher affinities calculated by the Langmuir equation for phosphate than glyphosate. In the competitive experiments, the order of glyphosate and phosphate addition affected the final sorption but in a complex way. Despite stronger bonding of phosphate than glyphosate, phosphate could only displace small amounts of pre-sorbed glyphosate. In this case, the amounts of sorbed glyphosate and phosphate were nearly additive. In contrast, pre-sorbed phosphate suppressed glyphosate sorption, in particular on the volcanic soil (Sasanda), maybe because sorbed phosphate increases the negative charge on the soil surface, making it more repellent to the negatively charged glyphosate molecules. Accordingly, additive as well as competitive sorption can dominate the reaction of variable-charge soils with glyphosate and phosphate.