Removal of Hydroxy- Interlayering from Soil Clay Smectites- An Improvised Method.

Abstract

Hydroxy-interlayering (HI) in smectites is a common phenomenon in shrink-swell soils of Peninsular India (Pal et al., 2003, Ray et al., 2008). The nature and chemistry of these HI materials have direct bearing on the physical and chemical properties of these soils (Deshmukh, 2009, Bhople, 2011). Hence these HI also affect the soil-plant relationship and thus the growth and management of these soils. Moreover, little is known about the interlayer composition of soil smectites from India and abroad in general, and Maharashtra state in particular for proper characterization and genesis of these soils for their use and management. Understanding the importance of such type of study, the present study has been taken up. Fine clay fractions of two benchmark soil series, viz., Asra and Nimone from Amravati and Ahmadnagar districts of Maharashtra were chosen for the study. Fine clay fractions were used for the study. Seven extractants viz., ethylene diamine tetra acetic acid (EDTA), sodium citrate (SC), sodium oxalate (SO), sodium tartarate (ST), sodium acetate (SA), ammonium oxalate (AO) and very dilute solution of HCl were used to remove hydroxy-interlayering from the interlayers of smectites. The fine clays were dominated by smectites with small proportions of vermiculite and chlorite. All the extractants were able to remove the HI in smectites to an appreciable extent thereby increasing the amount of smectites. However, in case of AO and HCl an opposite effect was observed, wherein the smectite content decreased drastically. The removal of HI was ascertaind by the higher intensity of the glycolated peak over the Ca-saturated peak and also narrowing of the K-saturated and heated peaks. The composition of the elements extracted were in the order Al~ Fe > Mg > Ca >> Mn for most of the extractants. However, AO and HCl extracted much higher values as it destroyed the smectite structure. Hence, AO and HCl are not suitable under the conditions used in this experiment. Among the other extractants, EDTA and SC extracted higher amount of the elements. XRD analysis substantiated these observations wherein the K treatments of fine clays showed much better removal of HI (pedogenic chlorites) compared to other extractants. It is also known that the interlayer Al(OH)3 / Fe(OH)3 /Mg(OH)2 sheets in HIS are more soluble than their separate phase counterparts (Kittrick, 1982) rendering the HI materials more soluble in the hexadentate structure of EDTA. Moreover, considering the chemistry of EDTA which forms strainless five member rings on chelation compared to other extractants, especially SC (which is more often used to remove amorphous oxides cemented in soils) and the various types of hydroxy-interlayered smectites, composition of HI materials, stability of EDTA-metal complex, etc., EDTA appeared to be more suitable option to remove HI from smectites of Peninsular India and also to ascertain its composition.