Oxalic Acid From Vegetable Waste

Published on 31/03/2014

First Update 15/03/2017

The vegetable processing industry generates significant quantity of wastes. There are several recycling programs but still tonnes of garbage are tossed out every day from the commercial establishments; a major chunk of this is vegetable waste matter and approximately 146.55 million tonnes of vegetable waste is generated in India after processing, distribution and consumption in an organized sector.  The majority of fruit and vegetable wastes like tomato pomace, bottle gourd pomace, citrus pulp, carrot pulp, baby corn husk and forage, cabbage and cauliflower leaves, sarson saag waste and pea pods, pineapple waste and pineapple bran etc. are perishable and highly fermentable, mainly because of their high moisture (80 to 90 per cent), crude protein (10 to 24 per cent) and total soluble sugars (6 to 64 per cent) contents. During the peak production or processing season, vegetable wastes are generated in huge quantities but are not consumed at the same pace as they become available and thus surplus quantities of these are discharged which can cause environmental pollution. These wastes usually contain high levels of organic matter, nutrients, moisture and sometimes salts and are not suitable for disposal in municipal landfills because of their physical, chemical and biological properties. Land application is the method most widely used by the food processing industry for disposal of vegetable wastes.

What is Oxalic acid?

Oxalic acid is an organic compound with IUPAC name ethanedioic acid and formula H₂C₂O₄. In its solid form, it is colorless and has the appearance of a white crystal substance when purified. It occurs naturally in many plants and vegetables and metabolism of molds also results in its production. Tissues of many plants and algae contain oxalic acid, which serves to excrete and store calcium. In water, its negative ion forms complex with a number of metal ions and oxalic acid is immobilized as a result of this formation of complexes. Both aerobic and anaerobic conditions biodegrade oxalic acid in less than one day. Oxalic acid is also used as an indoor disinfectant but is degraded readily and rapidly under both anaerobic and aerobic conditions during sewage treatment. In case oxalic acid is detected in the environment, it is the result of natural processes and not due to use of the chemical as a disinfectant.

Humans in very small amounts often ingest oxalic acid because it is inherent in foods, particularly broccoli, cucumbers, potatoes and sprouts. More concentrated amounts are present in dark, leafy greens in the form of oxalates. These foods are beneficial to health in small amounts. Foods such as spinach and broccoli are the most common sources of these types of oxalates (Table 1).

Table 1: Oxalic acid content in different vegetables

Oxalic acid is exempted from any tolerance when it is used as an inert ingredient in pesticide formulations that are applied for cultivating crops. The use of oxalic acid for oral and topical applications in humans is highly toxic to the body due to its bleach-like and corrosive properties. These properties are highly useful for general cleaning and waste water applications and this acid is therefore commonly employed for these purposes. Oxalic acid is used as an automobile radiator cleanser, general metal and equipment cleaner, in leather tanning and as a purifying agent and intermediate for many compounds.

Environmental impact of oxalic acid containing vegetable waste

The short-term effects of utilization of these wastes for land application can be odor and vector attraction (e.g., insects and rates) and random changes in salinity and soil pH. Long-term effects are likely to be on and off-site effects caused by salt accumulation in soil, nutrient leaching and nutrient imbalance. Soil salinity and sodicity caused due to high levels of sodium salts induced by waste application may damage soil structure as well as reduce crop growth. Nitrogen is the most commonly monitored nutrient associated with land application of vegetable wastes. To minimize nitrate leaching and contamination of water bodies, it is important that the application rate should be based on crop nitrogen requirements, i.e., based on an agronomic rate. If waste is applied to a soil with high phosphorus level and in a sensitive environment (e.g., close to significant fresh water lakes), phosphorus loading also needs to be taken into account.

Manufacturing process of oxalic acid from wastes

Two processes are widely known to extract oxalic acid from vegetable waste.

A)        The vegetable wastes are digested to form slurry that contains oxalic acid. The slurry is filtered and 4 per cent lime solution is added to the filtrate to completely precipitate calcium oxalate. The calcium oxalate, thus formed, is separated by filtration and dissolved in sulfuric acid, which causes precipitation of calcium sulfate that is further filtered. The filtrate is transferred to drying process to get oxalic acid crystals.

B)        The vegetable waste is collected in a digester and 1 per cent sulfuric acid is added to it (on a dry weight basis). This is heated to 110 degree Celsius (or 15 lbs/sq. inch pressure) to digest the vegetable waste to form slurry. Further, cellulose enzyme along with L amylase is added to the slurry to convert all the cellulosic material in to monosaccharide; this reaction will be conducted at 60 degree Celsius at 20 to 30 hours, after which the sugar can be converted to oxalic acid. Nitric acid (1.6 times of the dry weight basis of sugar) is added to the solution and compressed air is passed (@ 1 volume per volume of media) at 60 to 65 degree Celsius for 4 to 5 hours to convert sugar to oxalic acid. The oxalic acid, thus formed, is precipitated by adding lime solution. The solution is filtered to separate calcium oxalate, which is dissolved in dilute sulfuric acid. Furthermore, filtration is performed to separate oxalic acid crystals and calcium sulfate.

About 25 per cent of produced oxalic acid is used in dyeing processes as a mordant. It finds its application in bleaches, especially for pulpwood. It is also used in baking powder and as an important reagent in lanthanide chemistry. Hydrated lanthanide oxalates readily form strongly acidic solutions in an easily filtered form. These oxalates on thermal decomposition give their oxides. Oxalic acid is also rubbed onto marble sculptures to result in a shining surface.