Accelerated spent coffee grounds humification by heat/base co-activated persulfate and products’ fertilization evaluation

Abstract

Coffee, as the second most common beverage in the world, produced 60% of spent coffee grounds (SCG) with per ton of coffee beans processed. SCG is a typical lignocellulosic-rich organic waste and mainly disposed via composting or incineration. In this study, a rapid humification approach was proposed for the recycling of SCG using heat/base co-activated persulfate (heat/KOH/PS) advanced oxidation process. The yields of humic-like acid (HLA) and fulvic-like acid (FLA) reached 45 (3.96%) and 192 mg/g (19.2%) under the optimal humification conditions of 1% PS and 4% KOH at 100 ℃ in 1 h. The typical active groups of -OH and -COOH in FA standard were observed with higher amounts in the product compared to SCG, which may be related to occurrence of hydroxylation, carboxylation and Maillard reactions during humification. Radicals of •OH and SO4•- were identified in heat/KOH/PS system and made significant contribution to SCG humification. A slow-release nano FLA fertilizer (SNFF) was prepared by mixing treated SCG with attapulgite and showed good slow-release behaviors of HLA and FLA. In pot experiments with acid soil, SNFF increased the average root length of chickweeds by 233% compared with blank. Meanwhile, SNFF also contributed to increased abundance and richness of soil microbial community as well as a pH rise from 5 to 6.7, which was conducive to acid soil amendment. The earthworm test indicated positive ecological safety of SNFF. Overall, this study highlights an efficient humification method for the recycling of organic biowaste such as SCG in green agriculture.