Oxalic acid weathering

Acidolysis is a similar weathering reaction to hydrolysis in that is used to weather minerals, but in this case the source of is not water but organic or inorganic acids. Humic and fulvic acids (discussed in Section 8.3.2), carbonic acid, nitric or sulfuric acid, and low-molecular-weight organic acids such as oxalic acid can all provide H to weather minerals. All of these acids occur naturally in soils in addition nitric and sulfuric acid can be added to soil by acid pollution. The organic acids are prevalent in the... 

Experimental studies of mineral weathering rates in the presence of oxalic acid demonstrate the importance of LPD. For example, in the presence of 1 mM oxalic acid, rates of silica elution from feldspar can increase up to 15-fold at circumneutral pH, while A1 elution rates can increase by two orders of magnitude (Barker et al, 1997). Similar results are reported for quartz and olivine (Grandstaff, 1986 Bennett et al, 1988), and indicate that oxalate leaching of aluminium, calcium, magnesium and other cations from primary silicate minerals can yield a silica-rich residue similar to that found in association with endolithic lichens (Johnston Vestal, 1993 Lee Parsons, 1999). 

Most weathering processes, to be sure, take place in an environment where aerobic bacteria are active, unless there are considerable quantities of nitrogenous organic matter present. Anaerobic conditions, which usually also imply pH values close to (or less than) 7, are encountered in certain marine muds, guano deposits, and elsewhere. The anaerobe, Clostridium acidiurici (Liebert), which is known to occur in soils, produces ammonia, carbon dioxide and acetic acid from uric acid, guanine and xanthine (Baker and Beck, 1942), while Streptococcus allantoicus forms — in addition to ammonia and carbon dioxide — urea, oxamic acid, etc. Oxalic acid is a common product in the decomposition of guano. 

Lichens produce a variety of chemical compounds which are of potential importance in the biogeochemical weathering of minerals and rocks. Of these compounds, carbon dioxide (CO2), oxalic acid, and a large group of substances frequently referred to as lichen acids, but called lichen compounds herein, are the most significant. 

The importance of oxalic acid in the chemical weathering of minerals and rocks was emphasized by several early workers, Salter (1856) suggested... 

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