Review: organic matter removal from soils using hydrogen peroxide, sodium hypochlorite, and disodium peroxodisulfate

We compare the performance of three most accepted reagents for organic matter removal: hydrogen peroxide (H 2 O 2 ), sodium hypochlorite (NaOCI) and disodium peroxodisulfate (Na 2 S 2 O 8 ). Removal of organic matter from soil is mostly incomplete with the efficiency of removal depending on reaction conditions and sample properties. Generally, NaOCI and Na 2 S 2 O 8 are more effective in organic C removal than H 2 O 2 . Alkaline conditions and additives favoring dispersion and/or desorption of organic matter, such as sodium pyrophosphate, seem to be crucial for C removal. Pyrophosphate and additives for pH control (bicarbonate) may irreversibly adsorb to mineral surfaces. In soils with a large proportion of organic matter bound to the mineral matrix, for example subsoils, or rich in clay-sized minerals (Fe oxides, poorly crystalline Fe and Al phases, expandable phyllosilicates), C removal can be little irrespective of the reagents used. Residual organic C seems to seems to represent largely refractory organic matter, and comprises mainly pyrogenic materials and aliphatic compounds. If protected by close association with minerals, other organic constituents such as low-molecular weight carboxylic acids, lignin-derived and N-containing compounds may escape chemical destruction. For determination of mineral phase properties, treatment with H 2 O 2 should be avoided since it may promote organic-assisted dissolution of poorly crystalline minerals at low pH, disintegration of expandable clay minerals, and transformation of vermiculite into mica-like products due to NH + 4 fixation. Sodium hypochlorite and Na 2 S 2 O 8 are less harmful for minerals than H 2 O 2 . While the NaOCI procedure (pH 9.5) may dissolve Al hydroxides, alkaline conditions favor the precipitation of metals released upon destruction of organic matter. Prolonged heating to >40°C during any treatment may transform poorly crystalline minerals into more crystalline ones. Sodium hypochlorite can be used at 25°C, thus preventing heat-induced mineral alteration.