Pb/Ca ion exchange on kaolinite clay modified with phosphates

Abstract

Pollution of soils by heavy metal ions has attracted global concern because of the subsequent translocation into food chain which when taken up to a certain level can cause serious health problems. The influence of preadsorbed calcium by kaolinite clay modified with orthophosphate and tripolyphosphate reagents on the mobility of Pb2+ in kaolinitic soil system is studied. This is with the view to understand the fate of Pb2+ in phosphate-fertilized kaolinitic soils that are subsequently limed. Potassium dihydrogen phosphate (98%) and sodium tripolyphosphate (87%) were purchased from Aldrich and Merck, respectively. They were used to modify locally obtained kaolinite clay. These adsorbents were further pretreated with 0.1 M Ca(NO3)2. These phosphate-modified kaolinite clay adsorbents pretreated with Ca2+ were used in the adsorption of Pb2+ from aqueous solution in a batch mode. X-ray diffraction spectra of the various adsorbents indicated that modification of kaolinite clay with the reagents were effective on the surface of the clay mineral and not on the crystal structure of the clay mineral. Treatment with 0.1 M Ca(NO3)2 gave K–Ca (for unmodified kaolinite clay), K–O–Ca (for potassium-dihydrogen-phosphate-modified kaolinite clay), and K–TPP–Ca (for sodium-tripolyphosphate-modified Kaolinite clay) samples. The pHPZC of the adsorbents before pretreatment with Ca2+ decreased when compared with the unmodified kaolinite clay. Further pretreatment of these adsorbents with Ca2+ largely decreased the adsorption capacity of all three adsorbents for Pb2+ from aqueous solution due to the higher reactivity of Ca2+ compared with Pb2+. Data from all three adsorbents failed the nonlinear single Langmuir model fit while data from K–O–Ca adsorbent failed the competitive Langmuir model fit. ΔS o data were used to support the proposed structures of adsorbed Pb2+ on the various adsorbents. ΔH o for Pb/Ca exchange was endothermic in nature for K–O–Ca adsorbent and K–TPP–Ca adsorbents. However, it was exothermic for K–Ca adsorbent. ΔG o for Pb/Ca exchange on modified adsorbents was nonspontaneous but was spontaneous for K–Ca adsorbent. With increasing temperature, ΔG o for the ion exchange reaction became more spontaneous for all adsorbents. Pb/Ca ion exchange on phosphate-modified kaolinite clay adsorbents is endothermic in nature and nonspontaneous. However, with increase in temperature, spontaneity of the ion exchange reaction increased. This study suggests that liming of kaolinitic soils will decrease strongly the adsorption capacity of the soil for Pb2+, especially when they have initially adsorbed phosphates.