Waste milk humification product can be used as a slow release nano-fertilizer

Global milk production is accompanied by substantial waste milk (WM) generation, with prior estimates indicating ~100 million tons wasted in 2009. Conventional disposal to water bodies or sewage systems contributes to eutrophication, greenhouse gas emissions, and resource loss. Although WM is rich in carbohydrates, proteins, and lipids and could be a potential soil amendment, direct application leads to fermentation in soil and reduced germination. The study hypothesizes that humifying WM into stable humic-like acid (HLA) and fulvic-like acid (FLA) can stabilize and enrich its organic components for agricultural use. Existing microbial humification is slow (weeks) with low yields, while abiotic methods often require high temperature and pressure. The research question is whether a heat-free, rapid, base-activated persulfate (PS) process can humify WM into HLA/FLA efficiently and whether the resulting product, formulated as a slow-release nano-fertilizer with attapulgite, can promote plant growth and amend acidic soils.

Artificial humification transforms biomaterials into humic substances via microbial or abiotic processes. Microbial routes yield low HLA/FLA over weeks. Abiotic methods (e.g., hydrothermal, catalytic oxidation) accelerate humification to hours but at 180–250 °C and elevated pressures, increasing energy use. Humic and fulvic-like acids are recognized for improving soil fertility and crop productivity; FLA, with lower molecular weight and higher solubility, is particularly bioactive. Base-activated persulfate generates reactive oxidative species (•OH, SO4•−) effective in degrading organics and initiating radical polymerization—central to humification. Prior composting of food wastes typically achieves low HLA (2.6–8%) and FLA (0.7–4%) with higher carbon losses (30–60%). The study builds on persulfate-initiated polymerization chemistry and aims to apply it to concentrated organic matrices like WM to achieve rapid, ambient-condition humification with higher yields.

- Waste milk humification: PS (K2S2O8) and KOH dosed at 2–6 g each into 50 mL WM at 22 ± 1 °C with stirring. Time-course sampling at 0–120 min. Optimization of PS/KOH dosages and reaction time using 3D excitation–emission matrix (3D-EEM) fluorescence with fluorescence region integral (FRI) analysis to monitor shifts from protein/SMP to FLA/HLA regions. UV–vis corroboration. - Quantification of HLA/FLA: Modified BS ISO 19822-2018 protocol. At pH ~10.5, separation of non-humic sediment and soluble humic acids (SHA). Acidification to pH 1 to separate HLA (precipitate) and FLA (supernatant). Dry weights used to calculate contents via mass-balance equations, adjusting for K2SO4 formed by complete PS decomposition and minor ash (<0.3%). FLA purified by DAX-8 adsorption, NaOH desorption, and protonation on IR120 resin. - Radical identification and role: Electron paramagnetic resonance (EPR) with DMPO spin-trapping to detect •OH and SO4•− at 2 and 30 min. Quenching experiments with tert-butanol (•OH scavenger) and ethanol (scavenges •OH and SO4•−) to assess effects on EEM shifts (precursor degradation/humification). Temperature monitoring to assess exotherm. - Molecular characterization: FTIR, solid-state 13C NMR, and XPS to compare functional groups and carbon speciation of WM, product, and FA standard; GPC for molecular weight distribution (filtered samples). - Fabrication of slow-release nano fulvic-like acid fertilizer (SRNFF): Mix 50 mL liquid product (from optimal condition: 4 g PS + 4 g KOH, 1 h) with 25 g attapulgite (ATP) and 0.1 g xanthan gum; granulate into ~3 mm spheres; air-dry to ~30% moisture; dip-coat in 100 mL aminosilicone oil for 1 min; air-dry to yield 55 ± 0.1 g SRNFF. Morphology via SEM; XRD for crystalline phases; FTIR for interaction (hydrogen bonding/intercalation) between ATP and humic/fulvic species. - Slow-release tests: 30 g SRNFF in 1 L water at initial pH 3 or 7. Periodic sampling until equilibrium (96–120 h). Measure SHA at 267 nm and FLA at 241 nm, using pH-dependent standard curves for SHA and pH-independent for FLA. Track solution pH during release. Kinetic modeling including first-order model. - Pot experiment: Acidic soil (pH 5.0) pots (400 g total mix: 120 g soil + 280 g sand) with 8 chickweed seeds per pot, seven treatments in triplicate: Control (soil), WM (1.4 mL), ATP (0.7 g), K2SO4 (0.14 g; equal K content to product dose), COF (1.5 g), Product (1.4 mL), SRNFF (1.5 g containing 1.4 mL product + 0.68 g ATP). Watered every 3 days. After 21 days, measure fresh/dry weight, plant height, root length, leaf area, germination rate, chlorophyll content. Soil analyses: pH, TN, TP, TK, available N (AN), P (AP), K (AK), SOM. Statistics via Student’s t-test and one-way ANOVA with Tukey’s HSD (p < 0.05).

- Optimal humification at 4 g PS + 4 g KOH for 60 min at 22 °C increased fluorescence associated with FLA (region III) and HLA (region V), with FRI parameters PIII = 29.5% and Py = 32.3%. - Product composition (by weight method): HLA 44.4 ± 0.2% and FLA 25.5 ± 0.4%, substantially higher than typical composting yields (HLA 2.6–8%, FLA 0.7–4%). Organic carbon loss during process: 20.5% (lower than composting’s 30–60%). - Radical mechanisms: EPR detected •OH and SO4•− at 2 and 30 min. Quenchers (TBA and EtOH) suppressed degradation/transformations of proteins/amino acids (less shift in EEM), implicating radicals—particularly •OH—in initiating degradation, oxidation (hydroxylation, carboxylation), and polymerization (e.g., Maillard reaction). - Exothermicity: Reaction temperature in WM rose to ~61 °C within 10 min, then decreased as PS was consumed (68.4% within first 10 min; to 15.6% at 30 min), consistent with exothermic radical polymerization and PS activation. - Structural characterization: FTIR showed increased C–N, N–H, aromatic –OH; decreased aliphatic –OH and aldehyde C=O, consistent with Maillard condensation, dehydration, and cyclization. 13C NMR indicated higher alkyl (33–35 ppm), emergence of aromatic (130 ppm), and increased carboxyl (170 ppm) carbons. XPS showed decreases in C=O and increases in C–O and O–C=O, evidencing hydroxylation and carboxylation. - Molecular weight distribution: GPC shifted most intense distribution from ~1000 Da (WM) to ~500 Da (product), aligning with reported FA models (397–1203 Da), supporting high FLA content. - SRNFF formulation: SEM revealed product distributed within/onto ATP nano-rod porous networks. FTIR/XRD suggested physical interactions (hydrogen bonding between ATP Si–O–Si and product N–H; intercalation) and presence of K2SO4 alongside amorphous humic/fulvic phases. SRNFF sphere diameter ~3 mm; ATP rod diameter ~30 nm. - Loading and release: Loadings in SRNFF: SHA 103.3 mg/g; FLA 72.9 mg/g. At pH 7, equilibrium by ~96 h with cumulative release concentrations RC_SHA 1748 mg/L (RR 56.4%) and RC_FLA 1475 mg/L (RR 80.2%). At pH 3, slower release (equilibrium ~120 h), RC_SHA 1599 mg/L and RC_FLA 1317 mg/L; released fraction predominantly FLA (80.2% at pH 7; 71.6% at pH 3). Solution pH rose to 9.6 (pH 7 start) and 8.5 (pH 3 start). FLA release followed first-order kinetics (R2 > 0.99). - Pot trials (21 d, chickweed, acidic soil pH 5.0): Fresh weight per pot (mean ±): Control 1.05 g; WM 0.99 g (−5.4%); ATP 1.13 g (+7.3%); K2SO4 1.15 g (+9.4%); COF 1.24 g (+18.5%); Product 1.49 g (+41.9%); SRNFF 2.19 g (+109%) (all p < 0.05 vs Control except WM). Germination rate: 100% for Product and SRNFF, higher than other treatments. Leaf area increased by 42.1% (Product) and 104% (SRNFF) vs Control (p < 0.05). Taproot length: Control 6.39 cm; ATP 8.58 cm (+38.6%); Product 7.98 cm (+24.9%); SRNFF 10.11 cm (+58.2%) (p < 0.05). Soil pH increased to 7.3 (Product), 7.1 (SRNFF), 7.4 (ATP). Available K increased from 96.12 mg/kg to 232 (Product) and 237 mg/kg (SRNFF) (p < 0.05). - Cost and environmental aspects: Estimated total operation cost comparable or slightly lower than composting (256 vs 260 RMB per ton organic fertilizer). Process reduced carbon loss by ~62% relative to composting (based on 20.5% TOC loss) and generated recoverable heat (8.4 kJ/ton). Product EEM stability persisted for at least 12 days.

The findings demonstrate that alkaline-activated persulfate can rapidly humify waste milk at ambient conditions by generating •OH and SO4•− that degrade macromolecules and promote radical-induced polymerization (including Maillard-type reactions), forming humic/fulvic-like acids enriched in active functional groups. Structural analyses corroborate increased hydroxylation and carboxylation and the presence of typical FA-like features. Formulating the liquid product with attapulgite into a slow-release nano-fertilizer enhances nutrient utilization via hydrogen-bonding/intercalation-mediated adsorption, achieving controlled release that favors FLA availability. Pot experiments show that both the product and SRNFF markedly improve germination, biomass, leaf development, root elongation, and ameliorate acidic soil pH while boosting available K, outperforming conventional inputs (K2SO4 and compost) under the tested conditions. These results directly address the research aim by converting problematic waste milk into an effective, slow-release bio-stimulant/nutrient source, providing agronomic benefits and potential environmental advantages (lower carbon loss, heat recovery). Broader relevance includes a scalable, heat-free humification route potentially applicable to other organic wastes and contributing to sustainable fertilizer supplies where natural humic substances are limited.

This study presents a rapid, heat-free artificial humification process using KOH-activated persulfate to convert waste milk into a product rich in humic-like (44.4%) and fulvic-like (25.5%) acids within 1 hour. The product, when formulated with attapulgite as a slow-release nano fulvic-like acid fertilizer, achieved controlled release (first-order kinetics), improved plant growth metrics (biomass, germination, leaf area, root length), and amended acidic soil pH while increasing available K. The approach offers cost and environmental advantages versus composting, with reduced carbon loss and potential heat recovery, and shows signs of industrial practicability. Future research should include field-scale trials across crops and soils, detailed mechanistic elucidation of radical-driven humification pathways and structure–function relationships, long-term soil health and microbiome impacts, optimization of formulation and release profiles, comprehensive life cycle cost and environmental assessments, and broader validation using diverse organic waste streams.

- Efficacy evaluated only in pot experiments with chickweed and one acidic soil type; no field trials yet. - Release behavior assessed in aqueous systems (pH 3 and 7), not directly in soil environments. - Mechanistic understanding is inferred from EPR/quenching and spectroscopy; detailed pathways and kinetics of humification/polymerization require further study. - Product stability assessed over 12 days (EEM), lacking long-term storage and performance data. - Potential variability of waste milk composition and scalability across diverse waste streams were not fully explored.