Green conversion of excess sludge to N-Ca self-doping sustainable carbon quantum dots with remarkable fluorescence enhancement and residual heavy metal reduction

"Waste" is "Wealth". Recovering valuable materials from excess sludge (ES) is trending as an efficient strategy that synergizes waste management and resource recovery. Targeting the inherent composition characteristics of ES, this study aimed to explore resource recovery strategies that can exert its unique advantages. ES from paper mill was unprecedently selected as the carbon precursor, and ES-derived carbon quantum dots (ESCQDs) were successfully synthesized via a facile and one-pot hydrothermal reaction without pretreatments, catalysts or any additives. Notably, its fluorescence quantum yield (QY) and mass recovery rate were as high as 28.472 % and 2.86 %, respectively, which were at a high level among biomass-derived CQDs. N-Ca was successfully co-doped into ESCQDs without any nitrogen or calcium amendment, which has been reported rarely. The effect of Ca doping on the enhancement of QY of CQDs was revealed, thereby revealing the natural advantageous properties of ES from paper mill in the synthesis of high fluorescent Ca-N co-doped CQDs. Furthermore, the effect of the preparing ESCQDs on the environmental risks control of ES were also investigated. This resource recovery process efficiently removed residual heavy metals from ES and significantly reduced and controlled the environmental risk of heavy metals in ES, and the synthetic ESCQDs had low toxicity and excellent biocompatibility. This study proposed a green alternative that combines the recovery of value-added products from ES with heavy metal control, provided more in-depth understanding for the formation mechanism of CQDs, and revealed structural strategies for the preparation of high fluorescent CQDs.