NATURAL CHABAZITE-RICH ZEOLITIC TUFF FOR AMMONIUM UPTAKE FROM ANAEROBIC DIGESTATES

Nitrogen losses from anaerobic digestates represent a major environmental challenge and limit their efficient use as fertilizers [1]. Natural zeolites offer a promising solution for ammonium recovery through adsorption, enabling nutrient recycling and reduced emissions [2]. This study investigates the use of a chabazite-rich natural zeolitic tuff (~70% chabazite) for NH₄⁺ removal from real digestates derived from municipal solid waste, swine, and cattle manure. Adsorption kinetics and isotherms were evaluated considering digestate composition, solids content, pre-treatments, and competing ions. The approach considers the integration of the chabazite-rich tuff into a field-oriented prototype, designed to allow the direct reuse of the NH₄⁺-loaded material as a slow-release soil amendment within circular nutrient management strategies consistent with European Green Deal objectives. NH₄⁺ adsorption onto the tested zeolitic tuff showed a rapid initial uptake, with equilibrium reached within 90 minutes for SD-S, SD-M, and CD-S, and 120 minutes for MD-R and MD-C, highlighting the high ion exchange capacity of the material (Figure 1). The fast initial adsorption was attributed to the abundance of negatively charged surface sites and extra-framework cation exchange sites. Kinetic analysis revealed that the pseudo-first-order (PFO) model provided a better fit than the pseudo-second-order (PSO) model for all digestates, indicating that NH₄⁺ removal was primarily driven by ion exchange with readily available cations such as Na⁺ and K⁺. Intraparticle diffusion analysis showed three distinct stages: a rapid initial uptake due to surface adsorption, a slower phase controlled by internal diffusion, and a final equilibrium stage when all active sites were saturated. Increasing the solid-to-liquid (S/L) ratio enhanced the total NH₄⁺ removal due to the higher number of exchange sites, while qₑ values per unit mass of zeolite decreased. Removal efficiency followed the trend MD-C > MD-R > SD-S > CD-S > SD-M, reflecting the generally higher performance of municipal digestates compared to livestock-derived ones. Adsorption equilibrium data were well described by both Langmuir and Freundlich models, with the Freundlich model yielding slightly better fits (Figure 2), suggesting heterogeneous and multilayer adsorption behavior. Thermodynamic analysis (Table 1) confirmed that NH₄⁺ adsorption was spontaneous (ΔG° < 0) and favorable, particularly for municipal digestates. Pre-treatment of digestates, such as clarification, microfiltration, or screw-compression, further improved adsorption performance, leading to faster equilibrium, higher affinity, and greater overall NH₄⁺ reduction. The digestate composition strongly influenced adsorption efficiency: lower total solids (TS) and reduced concentrations of competing cations (particularly K⁺) favored NH₄⁺ removal, whereas higher TS and K⁺ levels hindered adsorption. Exceptions, such as the cattle digestate (CD-S), suggested that interactions among multiple cations also affect the process. Model-based estimations of farm-scale nitrogen recovery, assuming continuous operation with 10 m³/day of digestate and maximum S/L ratio of 15%, indicated that livestock-derived digestates could achieve up to 3000 kg N/year (Figure 3), while municipal digestates exhibited lower absolute recovery but higher percentages of NH₄⁺ reduction. Preliminary field tests conducted in May 2025 using microfiltered swine digestate (SD-M) at a 3% S/L ratio in a batch prototype confirmed laboratory findings, with ~10% NH₄⁺ removal per cycle and an estimated annual nitrogen recovery of 715 kg N/year. A slight increase in pH from 7.6 to 7.8 was observed, consistent with prior reports of mild alkalinization during zeolite treatment. Overall, these results demonstrate that NH₄⁺-enriched zeolitic tuff is an effective and robust adsorbent for digestate treatment and has strong potential for use as a slow-release fertilizer, supporting sustainable nitrogen management and partial replacement of synthetic fertilizers. Three real anaerobic digestates derived from municipal solid waste, swine, and cattle livestock were collected from full-scale biogas plants in Northern Italy and characterized for pH, electrical conductivity, total solids, nitrogen forms, and major competing cations. In this study, municipal digestates were denoted as MD-R (raw municipal digestate) and MD-C (centrifuged municipal digestate), swine digestates as SD-S (swine digestate – solid fraction) and SD-M (swine digestate – microfiltered), and cattle digestate as CD-S (cattle digestate – solid fraction). Different pre-treatment processes were considered, including screw separation, microfiltration, and centrifugation. A natural chabazite-rich zeolitic tuff (~70% chabazite), quarried from central Italy, was characterized for chemical and mineralogical composition, cation exchange capacity, and particle size distribution. Batch adsorption experiments were conducted at 25 °C to evaluate ammonium removal using equilibrium isotherms and kinetic tests at different solid-to-liquid ratios. Adsorption data were described using Langmuir and Freundlich isotherm models and pseudo-first-order and pseudo-second-order kinetic formulations. A model-based framework was employed to calculate potential daily and annual nitrogen recovery for a continuously operated, farm-scale prototype, which was further evaluated through preliminary field tests under real operating conditions. This study demonstrates that chabazite-rich zeolitic tuff is an effective and sustainable adsorbent for NH₄⁺ recovery from a range of anaerobic digestates, showing clear potential for farm-scale applications. Adsorption occurred via a multilayer, heterogeneous mechanism, mainly driven by ion exchange, with kinetics best described by a pseudo-first-order model. Digestate composition, including K⁺ concentration and total solids content, significantly influenced performance, with livestock-derived digestates achieving the highest ammonium recovery. Pre-treatment methods, particularly centrifugation, improved adsorption by increasing site accessibility. Preliminary field-scale trials with microfiltered swine digestate confirmed process feasibility, with a potential nitrogen recovery of ~715 kg N/year per plant. The NH₄⁺-saturated tuff can be applied directly as a slow-release soil amendment, providing additional agronomic value. Overall, these findings highlight the suitability of zeolite-based systems for circular nitrogen management in agriculture, enabling nutrient recycling, reducing reliance on synthetic fertilizers, and supporting more resilient and sustainable food systems in line with EU Green Deal objectives. References [1] G. Policastro and A. Cesaro (2023) Int. J. Environ. Res. Public Health, 20, 312. [2] V. A. Tzanakakis, N. Monokrousos, and T. Chatzistathis (2021) J. Soil Sci. Plant Nutr., 21, 2791–2802.