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A homoionic Cu 2+-montmorillonite dehydroxylated at 540 °C is subjected to the Hofmann-Klemen- effect. Cu 2+-ions migrate from interlayer positions into octahedral vacancies and cause a charge reduction. The amount of structural OH decreased after preheating and did not be entirely regained by rehydroxylation under water steam at 200 °C in stainless steel autoclaves. Inspite of this structure defect the CEC increased after steam treatment and Mg 2+ occurred exchangeable in interlayer positions. Thereby an undefined quantity of Cu 2+ -ions is still captured in pseudo-hexagonal cavities of the tetrahedral sheet and pretend a final charge reduction, which is to high. The real amount of migrated Cu 2+ and Mg 2+-ions was determined by chemical analyze after dispersing the clay in a Na2CO3 solution.
The sample under examination was a pure well crystallized montmorillonite < 2 µm from Linden (Bavaria, D). The structural formula was determined from the chemical analyze with respect to the CEC measurement: (Si3.95 Al0.05) (Al1.46 Fe0.18 Mg0.39) O10 (OH)2 Mg0.007 Cu0.178 (M=376.7 g/mol, CEC: 98 mäq/100g). The starting and the treated material was analyzed by CEC measurement, determination of exchangable cations, XRD, thermogravimetry combined with mass spectrometry and by chemical analyze.
The amount of structural OH decreased to 2.2% after preheating at 540 °C and increased after rehydroxylation to 3.6 %, which is 75 % according to the untreated clay. The CEC decreased from 98 meq to 29 meq/100 g. After steam treatment the sample regained a CEC of 39 meq/100 g and 23 meq/100g Mg 2+ occurred exchangeable in interlayer positions. The amount of exchangeable Cu 2+ (3 meq/100g) did not increase after steam treatment. After dispersing the clay in a soda solution and after washing salt free the CEC increased to 62 meq/100g. With this procedure not all Mg 2+ and Cu 2+-ions were replaced by Na +. 46 meq Na +, 7 meq Cu 2+ and 9 meq Mg 2+ were found exchangable. These data furnish the evidence of the chemical formula of the rehydroxylated, chargereduced Cu-montmorillonite: (Si3.94 Al0.06) (Al1.42 Fe0.18 Mg0.33Cu0.18) O10 O0.25(OH)1.5 Na0.17Mg0.017Cu0.013.
Emmerich, Katja and Kahr, Günter
Clay; ClayGroup; cis-vacant; montmorillonite; dehydroxylation; rehydroxylation