Display omitted •Microplastics (MPs) with adsorbed hazardous chemicals threat humans and fauna.•Photocatalytic degradation of MPs by C,N-TiO2 was proposed to fight MPs pollution.•Photocatalysis of HDPE MPs was studied at different pH and temperature values.•A combined effect of low pH and temperature resulted in 70 % of MPs’ degradation.•Photocatalysis of HDPE MPs in aqueous medium can reduce MPs marine pollution. Microplastics (MPs) are pollutants formed by plastics ≤ 5 mm and are present in marine and terrestrial environments. Due to their large surface to volume ratio and chemical surface properties, MPs adsorb hazardous chemicals from their surrounding environment. When MPs are consumed by fauna, they transfer those substances through the trophic chain. An essential issue of MPs is their disposal. Due to their size, the disposal methods commonly used for plastic items are not suited for MPs. Here, photocatalysis in an aqueous medium is proposed as an alternative to fight MPs pollution. Although the photocatalysis of MPs has been reported, the effect of operating variables in the process has not been investigated. To fill this gap, the impact of pH and temperature on the degradation process of HDPE MPs was investigated using C,N-TiO2 and visible light. Degradation was followed by mass loss, carbonyl index calculation and microscopy. It was found that photocatalysis at low temperature (0 °C) increases MPs’ surface area by fragmentation, and low pH value (pH 3) favours hydroperoxide formation during photooxidation. By using the design of experiments tool, it was demonstrated that there is a combined effect of pH and temperature in the photocatalysis of HDPE MPs.