Fractionation of waste-derived volatile fatty acids by multi-stage adsorption using activated charcoal and Diaion HP-20 resin

Abstract

Substituting waste-derived Volatile Fatty Acids (VFAs) with their conventionally applied fossil-derived counterparts in a spectrum of industrial applications necessitates its proper fractionation into individual acids. This study explored a multi-stage batch adsorption approach for fractionating acidogenic fermentation VFAs effluents from food waste (FW) and chicken manure (CKM) using Diaion HP-20 and activated charcoal. Initial screening at different washing conditions and pH (3.5 and 6.5) revealed the unwashed granular-activated charcoal (GAC-Unwashed) and milli-Q water-washed Diaion (DI-MQ Washed) as the most promising candidates for VFA fractionation of a synthetic VFA mixture at 4 gL^-1. At pH 3.5 ($<p{K}_a$), GAC-Unwashed adsorbed 2-6 carbon atom VFAs completely, while DI-MQ Washed exhibited minimal adsorption of acetic acid (AA) (8%), favoring caproic (CA) and valeric acids (VA) ($>$97%). While at pH 6.5 $(>p{K}_a$), GAC-Unwashed selectively targeted VA (79%) and CA (100%). Fractionating VFAs from FW and CKM were conducted in a two-stage adsorption process with optimal results being achieved using GAC-Unwashed at FW initial pH (5.3) and DI-MQ Washed at pH below CKM $p{K}_a$ (3.5), respectively. The first adsorption stage primarily adsorbed higher molecular weight (MW) VFAs (FW:99.1% CA, CKM:72.9% butyric acid (BA)) with a minor quantity of lower ones (FW:56.5% BA, CKM:29.3% propionic acid (PA)), leaving AA intact. Subsequent stages aimed to isolate AA by adsorbing the remaining low MW VFA (FW:58.9% BA, CKM:27.8% PA, 70% BA) other than AA, indicating effluent fractionation while preserving and purifying AA. Applied selective multi-stage adsorption approach offers a promising method to broaden waste-derived VFA applications.

Keywords: Adsorption; Diaion resin; Fractionation; Waste-derived volatile fatty acids; activated charcoal.

Graphical abstract

Figure 1.

Schematic of the experimental procedure used in this study (PAC: Powdered Activated Charcoal, GAC: Granular Activated Charcoal, DI: Diaion HP-20, MQ: Milli-Q water, ME: Methanol).

Figure 2.

Equilibrium adsorption percentage of MQ washed and unwashed granular and powdered activated charcoal (GAC and PAC) at two initial pH of 3.5 (a) and 6.5 (b).

Figure 3.

Variation of pH over time for MQ washed and unwashed granular and powdered activated charcoal (GAC and PAC) at two initial pH of 3.5 and 6.5.

Figure 4.

Equilibrium adsorption percentage of washed and unwashed Diaion HP-20 resin (DI) at two initial pH of 3.5 (a) and 6.5 (b).

Figure 5.

Variation of pH over time for washed and unwashed Diaion HP-20 resin (DI) at two initial pH of 3.5 and 6.5.

Figure 6.

Variation of VFAs concentration over time in synthetic food waste VFA effluent applying two different scenarios, (a) Step-1 of both scenarios using GAC-Unwashed-No pH adjustment, (b) Step-2 of the first scenario using GAC-Unwashed-No pH adjustment, (c) Step-2 of the second scenario using DI-MQ Washed at adjusted pH of 3.5.

Figure 7.

Variation of VFAs concentration over time in acidogenic fermentation VFA effluent derived from food waste applying two different scenarios, (a) Step-1 of both scenarios using GAC-Unwashed -No pH adjustment, (b) Step-2 of the first scenario using GAC-Unwashed -No pH adjustment, (c) Step-2 of the second scenario using DI-MQ Washed at adjusted pH of 3.5.

Figure 8.

Variation of VFAs concentration over time in the synthetic chicken manure VFA effluent applying three different scenarios. (a) Step-1 of Scenarios 1 and 2: GAC-Unwashed (at initial effluent pH, i.e. 6.2 ± 0.0), (b) Step-1 of Scenario 3: DI-MQ washed at pH 3.5, (c) Step-2 of Scenario 1: GAC-Unwashed (No pH adjustment), (d) Step-2 of Scenario 2: DI-MQ washed at pH 3.5, (e) Step-2 of Scenario 3: DI-MQ washed (No pH adjustment).

Figure 9.

Variation of VFAs concentration over time in the acidogenic fermentation VFA effluent derived from chicken manure applying three different scenarios. (a) Step-1 of Scenario 1&2: GAC-Unwashed (at initial effluent pH, i.e. 6.2 ± 0.0), (b) Step-1 of Scenario 3: DI-MQ washed at adjusted pH of 3.5, (c) Step-2 of Scenario 1: GAC-Unwashed (No pH adjustment), (d) Step-2 of Scenario 2: DI-MQ washed at pH 3.5, (e) Step-2 of Scenario 3: DI-MQ washed (No pH adjustment).