Objectives The bioactivity of humic acid is closely related to its carboxyl content. In this paper, the optimal processing parameters of increasing the humic acid carboxyl group content with hydrogen peroxide as oxidant were studied by using the method of response surface simulation.

Methods Weathered coal humic acid was used as raw material and 6% hydrogen peroxide as oxidant, the oxidantion process of humic acids was conducted in a three-factorial experiment, each factor having four-levels. The pH during the reaction were 1, 4, 7 and 10 under liquid/solid ratio of 0.2∶1, 0.4∶1, 0.6∶1, 0.8∶1, and reaction time of 60, 120, 180 and 240 min. The peak area of carboxyl group in the reaction products was determined using Fourier Transform Infrared Spectrometer and calculated by Origin 2018. According the single factor experimental result, the response of surface methodology (Box-Benhnken) test was used for the optimization of the reacting parameters.

Results In the single pH experiment, the highest humic acid carboxyl content was obtained in solution pH 7, which was 192.2% higher than that in untreated humic acid. In the reaction time experiment, the highest carboxyl content was obtained in 120 min. Then followed by 180 and 240 min during which the carboxyl contents were 83.0% and 48.5% higher than that in untreated humic acid. In the response surface optimization test, the model was highly significant, and the established regression equation for carboxyl content had an excellent goodness of fit. Therefore, the humic acid carboxyl content could be analyzed and predicted by the model. The effects of size of each independent variable on the carboxyl group content were shown in the following descending order: extraction pH > liquid/solid ratio > reaction time. The response surface plots indicated that solution pH had a significant effect on the humic acid carboxyl content, while the interaction between any two independent factors was not significant.

Conclusions pH is the most important factor influencing the carboxyl content during the oxidation of humic acids using H₂O₂ as dissolvent. Under the reaction temperature of 100°C, the optimum reaction parameters to increase the humic acid carboxyl content are: pH 7.49, reaction time 114.84 min, liquid/solid ratio 0.39∶1, and under this optimum reaction condition, the error rate between the predicted and actual value of carboxyl content is only 1.4%.