As the temperature gradually increase during biochar preparation, then why the pH is also constantly increased with the increasing temperature?
The carbon content in the biochar increased from 45.5% to 64.5% with increasing pyrolysis temperature, whereas the oxygen content decreased corresponding to an increase in the carbon content. This finding shows that carbonization was promoted with increasing pyrolysis temperature.[Chen Y , et al. ,2012] Hydrogen and oxygen losses at high pyrolysis temperature were due to the cleavage and breakage of weak bonds within the biochar structure. [Demirbaş A, 1997]
In pyrolysis process the carbon content in the biochar increased from 45.5% to 64.5% with increasing pyrolysis temperature, whereas the oxygen content decreased corresponding to an increase in the carbon content. Biochar reduces soil acidity which decreases liming needs, but in most cases does not actually add nutrients in any appreciable amount. Biochar made from manure and bones is the exception; it retains a significant amount of nutrients from its source. Because biochar attracts and holds soil nutrients, it potentially reduces fertilizer requirements. As a result, fertilization costs are minimized and fertilizer (organic or chemical) is retained in the soil for longer. In most agricultural situations worldwide, soil pH (a measure of acidity) is low (a pH below 7 means more acidic soil) and needs to be increased. Biochar retains nutrients in soil directly through the negative charge that develops on its surfaces, and this negative charge can buffer acidity in the soil, as does organic matter in general.CEC stands for Cation Exchange Capacity, and is one of many factors involved in soil fertility. “Cations” are positively charged ions, in this case we refer specifically to plant nutrients such as calcium (Ca2+), potassium (K+), magnesium (Mg2+) and others. These simple forms are those in which plants take the nutrients up through their roots. Organic matter and some clays in soil hold on to these positively charged nutrients because they have negatively charged sites on their surfaces, and opposite charges attract. The soil can then “exchange” these nutrients with plant roots. If a soil has a low cation exchange capacity, it is not able to retain such nutrients well, and the nutrients are often washed out with water.Most biochar trials have been done on acidic soils, where biochars with a high pH (e.g. 6 – 10) were used. One study that compared the effect of adding biochar to an acidic and an alkaline soil found greater benefits on crop growth in the acidic soil, while benefits on the alkaline soil were minor. In another study, adding biochar to soil caused increases in pH which had a detrimental effect on yields, because of micronutrient deficiencies which occur at high pH (>6). Care must be taken when adding any material with a liming capacity to alkaline soils; however, it is possible to produce biochar that has little or no liming capacity that is suitable for alkaline soils.
Dear Nasir, this article may help you.
https://www.researchgate.net/publication/262110040_Production_and_characterization_of_biochars_from_agricultural_by-products_for_use_in_soil_quality_enhancement/figures?lo=1
Article Production and characterization of biochars from agricultura...
Dear Nasir, Here we can mention two or three factors to play the role in increasing biochar pH with the pyrolysis temperature.
First, the ash content of the biochar increases with an increment in Pyrolysis temperature, which potentially increases pH.
Second, the Carbonates (especially Ca and Mg carbonates) increase with the increment of pyrolysis temperature.
Thirdly. the O containing acidic functional groups which contribute to the acidity of biochar at the lower temperature loses their oxygen at a higher temperature and this again increases pH of the biochar as the pyrolysis temperature increases.
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