The nature of soil particles particularly those reacting with arsenate in a series of chemically treated samples

Abstract

Arsenate uptake by a surface sample of a lateritic podzol was controlled mainly by iron oxide particles in a very finely divided state of about 50 A in diameter. Despite their size, these particles had some crystalline characteristics - they had semi-regular appearance; had limited solubility in ammonium oxalate (in darkness) and in sodium pyrophosphate; and sometimes gave ring-type electron diffraction patterns identifiable as goethite. The particles contained a high degree of substitution of aluminium for iron. They associated with other soil components, for example, forming surface deposits upon larger kaolin flakes or else microaggregates with smaller ones. They reacted with soil organic matter, which caused marked inhibition of arsenate uptake. Titanium oxides competed with iron oxides for arsenate, and were able to dominate uptake when iron oxides were removed chemically. The behaviour of other components towards arsenate was described, together with their appearance, composition and stability during a series of successive chemical treatments. No iron-organic complexes nor poorly crystalline aluminosilicates appeared to be present. Some reputedly selective reagents extracted excess aluminium, most of it coming from chloritized vermiculite. Small kaolin particles were also dissolved and, in some cases,. this caused fresh precipitation of iron oxides from iron previously held within the clay lattice.