Copper adsorption/desorption in phosphate- and sludge-treated oxisols: Kinetics and effects of aging andpH

Copper mobility in the environment typically is controlled by adsorption/desorption reactions. This research studied the kinetics and the effects of aging and pH upon Cu adsorption/desorption in phosphate- and sludge-treated A and B horizons of two Brazilian Oxisols. Phosphate fertilization is nearly ubiquitous in Oxisols. Sewage sludge application to agricultural land is increasing in Brazil. The time-dependence of Cu adsorption/desorption revealed that initial adsorption rate constants were up to 45% greater in P-treated, and up to 70% greater in sludge-treated than control soils. In P-treated soils, the fraction of Cu desorbed (Cu{dollar}rm sb{lcub}des{rcub}{dollar}/C{dollar}rm sb{lcub}ads{rcub}{dollar}) was half that in control soils, and smaller in B than A horizons. Sludge treatment affected Cu{dollar}rm sb{lcub}des{rcub}{dollar}/Cu{dollar}rm sb{lcub}ads{rcub}{dollar} somewhat less than did P treatment. Little Cu was desorbed from P-treated soils after 15 min, whereas desorption occurred up to 1 d for sludge-treated, and up to 18 d for control soils. Increasing adsorption reaction time (aging 1h to 54 d) increased Cu sorption distribution coefficients ({dollar}rm Ksb{lcub}ads{rcub}{dollar} and {dollar}rm Ksb{lcub}des{rcub}{dollar}); {dollar}rm Ksb{lcub}ads{rcub}{dollar} was 2 to 8 times greater for 54-d than 1-h aging, whereas {dollar}rm Ksb{lcub}des{rcub}{dollar} increased 2- to 4-fold over the same aging period. Aging caused up to an 80% decrease in Cu desorption. Samples aged 3 d or less exhibited rapid desorption followed by readsorption. {dollar}rm Ksb{lcub}ads{rcub}{dollar} and {dollar}rm Ksb{lcub}des{rcub}{dollar} values and aging effects on {dollar}rm Ksb{lcub}ads{rcub}{dollar} and {dollar}rm Ksb{lcub}des{rcub}{dollar} followed the trends P-treated {dollar}>{dollar} sludge-treated {dollar}>{dollar} control, 50 {dollar}>{dollar} 150 {dollar}mu M{dollar} Cu, and A {dollar}>{dollar} B horizon. Increasing pH from 4.5 to 6.5 increased adsorption up to 3-fold and decreased {dollar}rm Cusb{lcub}des{rcub}/Cusb{lcub}ads{rcub}{dollar} to {dollar}<{dollar} 0.01. At a given pH, more Cu was adsorbed on A than B horizons, and on pretreated than control samples. Copper desorption from aged soils can be overestimated more than 2-fold by measuring desorption after a 24- or 72-h adsorption reaction, as is typical in laboratory experiments. The large increase in initial Cu adsorption and the decrease in {dollar}rm Cusb{lcub}des{rcub}/Cusb{lcub}ads{rcub}{dollar} for P- or sludge-treated soils is noteworthy as this will affect Cu mobility and availability in Oxisols.