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From agricultural by-products to value-added materials: wheat straw-based hydrogels as soil conditioners? Katja Heise, Maximilian Kirsten, Yvonne Schneider, Doris Jaros, Harald Keller, Harald Rohm, Karsten Kalbitz, and Steffen Fischer ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00378 • Publication Date (Web): 28 Mar 2019 Downloaded from http://pubs.acs.org on April 3, 2019
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From agricultural by-products to value-added
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materials: wheat straw-based hydrogels as soil
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conditioners?
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Katja Heise†*, Maximilian Kirsten‡, Yvonne Schneider#, Doris Jaros#, Harald Keller, Harald
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Rohm#, Karsten Kalbitz‡, Steffen Fischer†
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† Institute of Plant and Wood Chemistry, Technische Universität Dresden, Pienner
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Strasse 19, 01737 Tharandt, Germany
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‡ Institute of Soil Sciences and Site Ecology, Technische Universität Dresden, Pienner
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Strasse 19, 01737 Tharandt, Germany
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# Chair of Food Engineering, Technische Universität Dresden, Bergstrasse 120, 01069
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Dresden, Germany
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* Corresponding Author: Katja Heise, E-mail:
[email protected] 13
BASF SE, Carl-Bosch-Strasse 38, 67056 Ludwigshafen, Germany
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KEYWORDS Biomass valorization, Moisture sorption, Soil amendment, Water retention
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ABSTRACT
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Herein, we present a simple synthetic approach to fabricate wheat straw-based
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hydrogels, starting from the unfractionated and carboxymethylated lignocellulosic
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matrix. Citric acid was used as a cheap and non-toxic crosslinker. The applied hydrogel
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characterizations can be essentially distinguished into investigations on the synthetic
20
pathway and model-scale application-related tests. For the first part, three sample-
21
specific values were introduced: gel yield (%), swelling ratio (gwater/g) and gel stiffness (G´,
22
Pa). Optimized reaction conditions led to mechanically stable gels with a moderate
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swelling ratio (up to 50 gwater/g). Moreover, dynamic vapor sorption analysis revealed
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that these gels re-swell after complete drying. Finally, one selected hydrogel was
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incorporated into two different model soil substrates, assessing its impact on the soils´
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water retention. Our experiments showed that already low incorporation rates
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(0.2 wt %) increased the water content of a sandy soil by 70 % (at pF 2.53). Overall, these
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results are promising and may lead to new soil amendments based on a sustainable
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source and a simple synthesis.
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INTRODUCTION
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Inspired by Nature´s zero-waste principle, the valorization of lignocellulosic biomass
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wastes has an enormous economic and ecological potential, as they are produced in
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excess, globally, and their recycling allows for circular production strategies in
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agriculture and forestry.1–3
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To develop individual value-chains for cellulose, hemicelluloses and lignin, recent
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biorefinery concepts prevalently target biomass fractionation using elaborate
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pretreatments entailing large expenses for chemicals and energy.4 Alternatively, starting
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from unfractionated lignocelluloses in simplified operations may considerably reduce
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processing efforts and increase the economic value of biomass residues for selected
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applications.2
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In this regard, two product groups have been in the focus of applied research
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recently:
lignocellulose-based
bioadsorbents
for
waste
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lignocellulosic fiber-reinforcements for composite materials.5,6 A third and very
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promising idea leads back to agriculture and considers applying lignocellulose-based
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hydrogels to soils as miniature water reservoirs and nutrient carriers.7 These three
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concepts have one essential principle in common: they use chemical and/or physical
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treatments to optimize inherent polymer characteristics of the biomass matrix toward
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the intended application.7–10
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water
treatment
and
3
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Linear and crosslinked polymers have widely been investigated as soil amendments
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to prevent soil erosion or for improving the soils´ water holding capacity.11,12 In many
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cases, superabsorbent polymers (SAPs) are thereby based on crosslinked polyacrylate or
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polyacrylamide, which were found to absorb enormous amounts of water (10-10,000
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g/g) and improve the availability of water and fertilizers in soils.13 However, despite the
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successful use of SAPs in horticultural industry, their high costs and hampered
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biodegradability have so far restricted an application on agricultural scale.13,14
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Lignocelluloses offer an ideal polymeric framework for the fabrication of hydrogels
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for soil usage owing to their inherent hydrophilic nature, biodegradability and
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modifiability. Moreover, compared to purely cellulosic substrates, residual lignin
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within the gel matrix might slow down biodegradation preserving the gels´ structural
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integrity over an adequate time period (e.g. one growth season).15,16 Nevertheless,
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synthetic concepts for lignocellulosic hydrogels are still scarce. In current strategies,
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lignocelluloses (e.g. cotton stalks, rice straw, sugarcane bagasse) basically serve as
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biopolymer backbone for the synthesis of acrylate-based graft copolymers.7,17–20 The
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obtained semisynthetic SAPs exhibited excellent water absorbencies (up to 350 g/g) and,
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therefore, improved soils´ water retention. Nevertheless, the use of acrylate-derived
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products may be questioned, as leaching and accumulation of potentially eco-toxic
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monomers in natural systems should be mandatorily avoided.21,22 Furthermore, a
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complete degradation of soil amendments should be guaranteed.
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Polycarboxylic acids may be a very promising alternative in this regard, as they were
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found to effectively crosslink cellulose without the need for initiators or organic
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solvents.23–25 Among them, citric acid has gained increasing interest for preparing
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cellulose-based films or superabsorbent hydrogels, taking into account its low price,
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natural origin and non-toxicity.26,27 Moreover, implementations are simple, as the
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esterification of cellulose follows an easy heat-induced mechanism: citric acid
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dehydrates over two reactive anhydride stages and thereby crosslinks adjacent cellulose
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chains.28
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Inspired by this simple approach, we used citric acid to crosslink unfractionated and
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carboxymethylated wheat straw, which represents a class of highly available biomass
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residues with an estimated annual production of above 150 million tons in the
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European Union (FAO, 2017).29 The obtained gels were analyzed extensively focusing
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on the synthetic pathway and their potential application as soil conditioners.
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MATERIALS AND METHODS
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Materials. Chopped wheat straw (WS; Agrargenossenschaft Rossau e.G. Germany)
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was ball milled (MM 400, Retsch® GmbH, Haan, Germany): 5 min, f = 20 rpm, median
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particle size 43 m. All chemicals were applied in analytical state. Deionized water was
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utilized for all experiments. For water retention tests, a sandy (from subsoil, soil depth
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> 40 cm, Duebener Heide, Germany) and a silty model soil substrate (from limestone
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quarry, soil depth = 300 cm, Ostrau, Germany) were collected, air-dried, sieved to < 2
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mm and stored at room temperature (RT). Soil characteristics are summarized in the SI
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(Table S1).
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Hydrogel preparation. Carboxymethyl (CM) intermediates were synthesized and
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purified as described previously.30 Milled WS was modified with sodium
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monochloroacetate (MCA) in isopropanol/NaOH (aq) as follows: (i) alkalization –
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1 hour, RT; (ii) carboxymethylation after MCA addition – 55 °C, 3 hours. Per mol
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anhydrous glucose units of cellulose (AGUcell, 48.9 % in wheat straw) 1.5, 3.0 and 6.0
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mol MCA were used for WS1, WS2, and WS3, resulting in carboxymethyl group
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contents of 1.75, 2.56 and 3.79 mmol/g, respectively. The product work-up was carried
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out by: decanting remaining medium, product dispersion in 100 mL water,
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neutralization with acetic acid, followed by disperser-assisted precipitation in 500 mL
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96-% ethanol and, finally, 5 repeated dispersion-precipitation steps in water/ethanol.
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Carboxymethyl group contents were determined by ICP-OES (Na determination,
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device: SPECTRO CIROSCCD, SPECTRO Analytical Instruments, Kleve, Germany)
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after dialysis against deionized water (membrane: SpectraPor® 3, MWCO: 3.5 kDa),
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lyophilization
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HNO3/HF/HClO4.30,31
and
microwave-assisted
decomposition
of
sample
aliquots
in
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After carboxymethylation, CM-intermediates were crosslinked with citric acid
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monohydrate. 2, 4, 8 or 16 wt % citric acid (w/w dry CM-intermediate) were dissolved
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in 45 mL deionized water and 5 g of dry CM-intermediate were suspended in the
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solution. The mixture was kept for 24 hours at RT to homogeneously distribute the
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crosslinker. Subsequently, the mixture was spread on a Petri dish (thin layer) and pre-
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dried at 60 °C for 24 hours, followed by crosslinking at temperatures of 120, 140 or
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160 °C for durations of 15, 30, 60 or 300 minutes. Afterwards, the product was gently
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crushed, sieved to a defined particle size (150 to 800 µm) and suspended in deionized
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water for 24 hours at RT. Finally, by-products and water-soluble fractions were
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removed by repeated water-washing and the purified gel particles were oven-dried at
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45 °C.
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Hydrogel characterization. Analytical characterizations of straw-based hydrogels
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were distinguished into investigations on the synthetic pathway, gel-moisture-
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interactions and model-scale water retention tests in soil substrates.
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ATR-IR spectroscopy. Attenuated total reflection (ATR) FT IR spectroscopy was
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performed with a Bruker Tensor 27 (Bruker Optik GmbH, Ettlingen, Germany), having
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a RT-DLaTGS detector, KBr beam splitter and Platinum ATR (A225) diamond cell. ATR
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spectra were measured as follows: range = 4000 – 400 cm-1, resolution = 4 cm-1, 400
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scans. Average spectra were formed (OPUS Ver. 6.5, Bruker) out of three
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determinations.
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Gel yield. The gravimetric gel yield (in %) after esterification with citric acid was
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determined in duplicate by suspending 0.1 g of dry and sieved (150-800 m) sample
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particles (before removing water-soluble fractions) in 10 mL deionized water for
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24 hours at RT. Water-soluble proportions were then removed by washing the swollen
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particles over a pre-dried (at 105 °C) and weighed glass fiber filter (WhatmanTM, GF6)
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with deionized water, followed by drying the loaded filter (105 °C) to mass constancy.
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The gel yield % is defined as: ((mtotal – mF) * 100 %)/ m0; with mtotal as total mass of
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recovered gel particles on the filter, mF as mass of the filter and m0 the initial dry mass
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(determined in duplicate by drying the samples to mass constancy at 105 °C).
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Swelling ratio. 0.1 g dry and sieved (150-800 m) gel particles were weighed into a
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heat-sealable teabag and were swollen in 700 mL deionized water for 24 hours at RT.
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The teabag was then removed from the water, unabsorbed water drained off for 10
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minutes, followed by weighing the swollen hydrogel within the bag. The swelling ratio
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(in gwater g-1, duplicate measurements) is defined as: (mtotal - mTB - WTB)/ mGEL; where mtotal
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is the total mass of swollen gel within the bag, mTB and WTB are mass and water uptake
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of the teabag, respectively, and mGEL the dry-mass related sample mass.
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Gel rheology. Rheological measurements were carried out in duplicate with sieved
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(150-800 m) and fully water-swollen (24 h/ RT) gel particles using an AR-G2 rheometer
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(TA Instruments, New Castle, USA) with parallel plates (d = 25.0 mm) at 23 ± 1 °C.
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Dynamic strain sweep tests were conducted at = 1.0 rad/s, in a strain range of 0.001 to
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1.0 (10 points per decade) to determine the linear viscoelastic (LVE) region. Dynamic
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frequency sweep tests ( = 0.1 to 100.0 rad/s, 10 points per decade) were performed at a
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strain of 0.002. The gel stiffness (G´ Pa) was taken from frequency sweep experiments
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at = 1.0 rad/s.32
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Dynamic vapor sorption analysis (DVS). Gel-moisture-interactions were investigated for
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the following selected gel samples: WS1-CA4%, WS3-CA4% and WS3-CA8%, based on
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WS1 (1.75 mmol –CH2COONa g-1) or WS3 (3.79 mmol –CH2COONa g-1), respectively;
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esterification: 140 °C, 30 min, 4 or 8 wt % citric acid (as indicated). The measurements
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were carried out in duplicate using a Q5000 SA dynamic vapor sorption analyzer (TA
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Instruments, New Castle, USA). Approx. 5 mg sieved (150-800 m) and preconditioned
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(10 d over P2O5) xerogel particles were loaded into a quartz crucible. Relative humidity
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(RH) in the sample chamber (25 °C) was adjusted by mixing continuous streams of dry
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and moisture-saturated nitrogen with continuous flow. Initial mass of the dry sample
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was determined at RH = 0 %. In the first adsorption cycle (Ad1), RH was increased to 90
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% in 10 % increments. At each RH step, equilibrium mass was taken when the relative
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mass change was < 0.01 % for 5 min. The second cycle comprised of stepwise desorption
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to RH = 0 % (De1), the third cycle another adsorption to RH = 90 % (Ad2).
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Sorption isotherms were obtained by plotting equilibrium moisture contents of the
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samples (Meq, mgwater g-1sample) against relative humidity (expressed as water activity aw
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-). Sorption hysteresis was calculated as absolute difference between desorption and
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first adsorption Meq. Deviations between the second (Ad2) and first adsorption Meq were
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calculated to examine structural alterations within the gel matrices. Moreover, for
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determining monolayer capacity (M0 mgwater g-1sample) of the samples, sorption data
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were fitted to the Guggenheim-Anderson-de Boer (GAB) model: cG, GAB monolayer
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sorption energy; kG, GAB multilayer sorption energy; aW, water activity = RH/100:33,34 Meq =
M0 × cG × kG × aw (1 ― kG × aw) (1 + (cG ― 1)kG × aw)
(1)
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Water-accessible surface area AG m2 g-1 was obtained by: N0, Avogadro constant
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(6.022 x 1023 molecules mol-1); am, area captured by a single water molecule at 25 °C (1.06
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x 10-19 m²); Msorb, molar mass of water (18 g mol-1):35 AG =
M0 × N0 × am Msorb
(2)
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Water retention in model soil substrates. In model-scale, the impact of one selected
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hydrogel (WS3-CA4% – based on WS3 (3.79 mmol –CH2COONa g-1), esterification:
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4 wt % citric acid, 30 min, 140 °C) on the soils´ water retention was investigated. Two
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defined model soil substrates were used, representing a sandy and a silty soil
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(characteristics in SI, Table S1). Dry and sieved (150 - 800 m) gel granules were
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incorporated homogeneously into the soil substrates at rates of 0.2 or 0.7 wt % (w/w
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soil). Approx. 10.0 g of each soil/gel mixture were placed in aluminum cylinders
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(2.8 x 3.0 cm, diameter/height) and saturated with deionized water for 24 hours at RT.
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Subsequently, the cylinders were placed on a ceramic plate and the pressure was
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reduced stepwise using a 100-centimeter water column. At each step (0, 10, 30, 40, 50,
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60, 80 and 100 cm, corresponding to a pF value of 0.00, 1.00, 1.48, 1.60, 1.69, 1.78, 1.90
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and 2.00) the samples were equilibrated for 24 hours, followed by recording their
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weight. The pF value is an expression of soil water tension (common logarithm of cm
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water column) describing the soil matrix potential (defined as energy required to move
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water against adsorptive and capillary forces). Afterwards, the samples were exposed to
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pF = 2.53 (water column: 338.8 cm) in a pressure chamber, again allowing an
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equilibration for 24 hours. Finally, pF curves were obtained from average values out of
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three replications and were compared to results of pristine soil substrates.
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RESULTS AND DISCUSSION
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Hydrogel synthesis and gel properties. Figure 1 represents the synthetic pathway
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comprising carboxymethylation and citric acid crosslinking, and the resulting ATR-IR
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spectra
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carboxymethylation introduced a strong signal at 1591 cm-1 (–COO- asymmetric
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stretching vibrations) into the biomass spectrum. The subsequent crosslinking reaction
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gave two new peaks of weak nature, attributable to newly formed ester bonds ((C=O):
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1728 cm-1, (C-O): 1230 cm-1). Besides, a significant alteration of the lignocellulosic
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matrix – particularly in the course of the carboxymethylation – became evident through
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a considerable decline of hemicellulose-derived acetyl signals ((C=O): 1733 cm-1; (C-
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O): 1228 cm-1). Our previous studies on the carboxymethylation of lignocelluloses
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already showed that the strongly alcoholic-alkaline medium (pH > 12) caused a partial
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extraction of hemicelluloses and lignin throughout the reaction.30 For instance,
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mercerization studies in isopropanol-NaOH mixtures showed that a typical
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carboxymethylation conditions (55 °C, 3 hours) would reduce lignin and hemicellulose
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contents to 15.1 % or 9.4 % from originally 22.8 % or 27.3 %, respectively. Nevertheless,
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following the synthetic path, all lignocellulosic constituents may be involved in the
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modification reactions as they bear accessible OH-groups.
of
the
modified
biomass
(band
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Accordingly,
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Figure 1. (A) Schematic representation of the synthetic path, and (B) ATR-IR spectra of
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(top to bottom) unmodified wheat straw, carboxymethylated straw (WS2) and (c) citric
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acid-crosslinked WS2 (16 wt % citric acid, 140 °C, 30 min).
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Synthesis parameters (covering: –CH2COONa content of CM-intermediates, reaction
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temperature and duration, and crosslinker concentration) of citric acid esterifications
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were evaluated by introducing three sample-specific values: gel yield (%), swelling ratio
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(gwater g-1) and gel stiffness (G´, Pa). Exact data for each parameter set are summarized in
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the SI (Table S2).
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Strain sweep experiments showed that the strain resistance of the gel network
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decreased with an increasing content of carboxymethyl groups in the biomass (WS3 >
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WS2 > WS1) (see SI, Figure S1). This implies a more pronounced network extension
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through electrical repulsion of charged carboxymethyl groups – a typical observation
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for polyelectrolyte hydrogels.37 Besides, reaction temperatures above 120 °C and citric
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acid concentrations of > 2 wt % significantly enhanced the strain resistance of the gels; a
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further increase in temperature and crosslinker amount, as well as long reaction
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durations (300 min) rather affected the overall gel stiffness G´ in the same strain range
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than the strain resistance of the network (SI, Figure S2). The frequency responses
231
revealed a dominant elastic behavior over the entire frequency range, indicating that the
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polymer networks were sufficiently crosslinked (SI, Figure S3).
233 234
Figure 2. Crosslinking of the carboxymethyl intermediates WS1, WS2 and WS3 –
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development of gel yield (◼), swelling ratio (○) and gel stiffness (◆, G´) as a function
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of the carboxymethyl group content.
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Besides the impact on gel stiffness, carboxymethylation essentially determined yield
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and swelling capacity of the hydrogels. Gel yield and stiffness decreased with
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increasing carboxymethyl content, whereas the swelling capacity was significantly
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enhanced (Figure 2). Following these trends, WS1-CA4% gave a high yield after
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crosslinking (78 %) and rigid hydrogel particles (G´ = 1558 Pa), with a poor swelling
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capacity (8 gwater g-1). Swelling increased from WS2-CA4% (38 gwater g-1) to WS3-CA4% (50
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gwater g-1) with the higher content of ionic groups and corresponding to the increased
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water-solubility of the carboxymethyl intermediates (WS2, WS3). However, the
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crosslinking reaction required free and accessible hydroxyl groups. Therefore, yield and
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gel stiffness decreased remarkably for the gel based on WS3 (45 %, 399 Pa).
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Nevertheless, the product still appeared as mechanically stable and, thus, as promising
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candidate for soil tests.
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The impact of the reaction setting on gel yield, stiffness and swelling was evaluated
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by esterifying WS2 applying various sets of reaction temperatures, durations and
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crosslinker quantities (Figure 3). The actual amount of citric acid (Figure 3(A)) was
252
thereby naturally the most decisive factor: products crosslinked with 2 wt % citric acid
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were of a very soft texture, whereas high crosslinker concentrations (8 or 16 wt %)
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resulted in brittle gels and poor swelling. As depicted in Figure 3(B) crosslinking was
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also reinforced through prolonged reaction durations. Considering both swelling and
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structural stability of the hydrogels, 30 minutes were found to be the ideal curing time.
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A shorter duration (here: 15 min), however, insufficiently crosslinked the biopolymer
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chains, resulting in a high proportion of water-soluble fraction. Elevating the reaction
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temperature from 120 to 160 °C (Figure 3(C)) increased the product yield clearly (48 to
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69 %), probably owing to an accelerated thermal degradation of citric acid to its reactive
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anhydride at temperatures above 153 °C.38 The stiffness of these gels, however,
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remained within almost the same range.
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Figure 3. Esterification of the carboxymethyl intermediate WS2 (2.56 mmol –
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CH2COONa g-1): gel yield (◼), swelling ratio in deionized water (○) and gel stiffness
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(◆, G´) as functions of (A) citric acid concentration, (B) reaction duration and (C)
267
temperature.
268
Overall, the ideal reaction setting was found to be: 140 °C, 30 minutes, 4 wt % citric
269
acid. Compared to citric acid gels based on pure carboxymethyl cellulose,27 swelling of
270
the straw gels was considerably lower indicating to remaining lignin-carbohydrate
271
interconnections. Also, gels obtained from lignocelluloses by graft-copolymerization, as
272
in studies of El-Saied et al. (e.g. 2000 and 2016)17,18, had a much higher water-uptake
273
(350 g/g), however, likely originating from the synthetic vinyl-based polymer part.
274
Moreover, it is questionable whether too high swelling is beneficial for the integrity of
275
the soils´ unique pore structure.
276
Gel-water-interactions. Dynamic vapor sorption (DVS) analysis delivers a valuable
277
insight into gel-water-interactions (e.g. accessibility of hydrophilic sites, swelling
278
behavior) by studying the response of an inititally dry sample (xerogel) to humidity
279
changes. In our study, DVS gave answers on two central questions: (i) How did the
280
synthetic path affect the interaction between lignocellulosic matrix and water? and (ii) Do the
281
gels retain their structural integrity upon the contact with water and in a sequenz of sorption-
282
desorption cycles? The later point thus gives vital information for a potential real
283
application, considering complete drying throughout the period of usage. To evaluate
284
both aspects, samples with different carboxymethyl contents and/or different network
285
densities were analyzed.
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Figure 4 shows adsorption and desorption isotherms of the gels. Throughout Ad1 and
287
Ad2, all samples clearly displayed an increasing Meq with the rising aw. The sigmoid
288
curve shape – having a significant upswing at high aw – corresponds to type II isotherms
289
(Brunauer´s
290
physisorption.33,40 The water uptake at aw ≤ 0.2 is due to monolayer adsorption41, which
291
was fairly low for all samples. This indicates a poor accessibility of hydrophilic sites and
292
weak adsorbent-adsorbate interactions at low vapor pressures.40 Intermediate
293
humidities (0.3 ≤ aw ≤ 0.7) provoked a more convex isotherm shape. In this stage
294
multilayer sorption proceeded after the monolayer was saturated with water41, and the
295
deviation between the samples´ sorption isotherms started to grow remarkably.
classification39)
indicating
unrestricted
monolayer-multilayer
296 297
Figure 4. Sorption isotherms of the first and second adsorption (Ad1/Ad2) and the
298
intermediate desorption (De1) of gels based on WS1 (1.75 mmol –CH2COONa g-1) or
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WS3 (3.79 mmol –CH2COONa g-1). (crosslinking conditions: 140 °C, 30 min, 4 or 8 wt %
300
citric acid)
301
At high water activities (aw 0.8-0.9), Meq of the straw gels increased rapidly, probably
302
following moisture penetration into the sample particles, which led to swelling and
303
plasticization of the rigid polymer network and a liberation of further hydrophilic
304
sites.41,42 The curve upswing in this stage can be clearly linked to the degree of
305
carboxymethylation (WS3 > WS1), with respect to ascent and final Meq. Furthermore,
306
comparing isotherms of WS3-CA4% and WS3-CA8% reveals that the higher crosslinking
307
density of the latter gel has not reduced water sorption until aw 0.9. On the contrary, the
308
moisture uptake of WS3-CA8% was slightly enhanced, though equlibrium swelling was
309
much lower (WS3-CA4%: 50 g/g, WS3-CA8%:12 g/g). This finding can be likely attributed
310
to the DVS methodology, in which a thermodynamic equlibrium of the samples´
311
swelling process was not accomplished.
312
Deviations between adsorption and desorption processes are characterized by
313
hysteresis loops, implying (irreversable) structural alterations within the sorbents´
314
domains following their interaction with water.40 Sorption hysteresis occurred
315
throughout the entire humidity range for each sample. The extent and exact position of
316
hysteresis maxima within the humidity range, however, obviously depended on
317
individual sample characteristics, coinciding, in particular, with the crosslinking
318
density. Gels crosslinked with 4 wt % citric acid showed a maximum of hysteresis at
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intermediate aw. WS3-CA8% evidently retained high amounts of moisture particularly at
320
low water activities (aw 0.1-0.4), pointing on an entrapment of water within the gel
321
network. This may be related to the affinity of water to fill pores, which increases with
322
the crosslinking density.42 However, it is further conceivable that citric acid
323
esterification increased the interaction with water molecules due to the introduction of
324
further OH-groups.
325
Deviations between Ad1 and Ad2, including shifted starting points and a reduced
326
moisture-absorbency at aw = 0.9, were most likely induced by structural alterations
327
during the first experiment cycle. Particularly, in the case WS3-CA8%, Ad2 started from
328
a significantly higher moisture level, which again indicates water entrapment.
329
Moreover, the fact that Meq,
330
humidity range implies that water penetrated more easily in Ad2. This effect may be
331
linked to the formation of new sorption sites with the irreversable cleavage of weaker
332
bonds (e.g. hydrogen bonds) during network extension in Ad1.42,43 High contents of
333
carboxymethyl groups would increase this effect as they promote swelling.
334
Furthermore, for WS1-CA4%, the final moisture load (at aw = 0.9) was slightly reduced,
335
which can be attributed to gel shrinkage during desorption entailing an irreversable
336
formation of hydrogen bonds (hornification).44,45 The icreasing carboxylate content from
337
WS1 to WS3, however, obviously mitigated this effect.
Ad2
exceeded Meq,
Ad1
for each sample in the intermediate
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To interpret gel-moisture interactions, sorption isotherms were fitted to the GAB
339
equation (SI, Table S3) followed by calculation of monolayer capacity and water-
340
accessible surface area (Figure 5). As indicated by the higher M0 and AG for WS3-based
341
gels, carboxymethylation increased the accessibility of sorption sites. The subsequent
342
crosslinking step enhanced the interaction with water, likely owing to the additional
343
introduction of OH-groups. As discussed previously, higher monolayer capacities
344
during De1 imply a hysteresis of sorption domains (i.a. entrapment of moisture).
345
346 347
Figure 5. Monolayer capacity M0 (blue bars/values) and water-accessible specific surface
348
area AG (◻, black values) obtained by GAB model fitting of sorption isotherms of first
349
and second adsorption (Ad1/Ad2) and intermediate desorption (De1). (Gels based on
350
WS1 (1.75 mmol –CH2COONa g-1) or WS3 (3.79 mmol –CH2COONa g-1), crosslinking
351
reaction: 140 °C, 30 min, 4 or 8 wt % citric acid)
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Although it was shown before that WS3-CA8% retained water at low vapor pressures
353
during De1, the GAB model fit gave significantly decreased M0 and AG for the
354
desorption cycle. Both parameters further decreased in Ad2. The same phenomenon can
355
be observed for WS1-CA4%. In both cases, sorption sites obviously became unavailable
356
for water binding, which might be explained by an irreversable formation of hydrogen
357
bonds during De1. In contrast, WS3-CA4% gave a steady increase in monolayer moisture
358
levels and water-accessible surface area.
359
Overall, the development of M0 and AG underlines the impact of each synthesis step
360
on water-matrix-interactions. Sorption processes probably also correlate with the
361
disintegration of the lignocellulosic matrix and decrystallization of the cellulose fraction
362
accompanying carboxymethylation. The presence of charged carboxylate groups promoted
363
network extension, which accelerated moisture penetration and clearly mitigated the
364
irreversable aggregation of biopolymer chains during drying. This ensures a re-
365
swellability of the gel network, for example, in a real soil application. Particularly, WS3-
366
CA4% showed a promising structural integrity upon the contact with moisture
367
suggesting its further evaluation in a model-scale soil application.
368
Water retention in soil substrates. Using two different model soil substrates, water
369
retention tests were conducted to give a more application-related assessment of the gel
370
performance. Based on promising previous results, the straw gel WS3-CA4% was
371
exemplarily selected. Furthermore, the model soils chosen – classified as a sandy and a
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silty substrate – provided us information about the practical gel application (covering
373
particle sizes, exchangeable cations, etc. – SI, Table S1). However, in Nature, mixtures of
374
both soil substrates may rather be found. Increasing the storing capacity of plant
375
available water in sandy soils, for instance, may enable agricultural cultivation on
376
formerly unusable areas and could be, therefore, an important contribution to securing
377
food supply. Silty soils, on the other hand, as valuable agricultural grounds, may
378
benefit from an improved water storing capacity, particularly, in view of climate change
379
scenarios.
380
Figure 6 demonstrates how the amount of water retained in the sandy or silty soil and
381
in soil/gel mixtures developed as function of the applied matrix potential (pF). This
382
relationship represents important soil-moisture characteristics, e.g. the plant water
383
availability depending on water content. For both model soils, the incorporation WS3-
384
CA4% significantly increased the water retention throughout the pF range, with a direct
385
correlation to gel incorporation rate and, additionally, soil characteristics. The starting
386
point of the assay (pF = 0) enables the examination of unaffected soil-gel-interactions,
387
without external impairments. At that point (as well as throughout the pF-range), the
388
silty substrate inherently retained more water, owing to its smaller grain sizes and
389
consequently higher volume of medium sized pores compared to the sand (SI, Table
390
S1). However, swelling of the gel was obviously less restricted in the sandy soil,
391
resulting in a more remarkable improvement of the sands´ water retention. The
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deviation between the water content retained within the different soil/gel mixtures
393
became more evident with the increasing matrix potential and incorporation time. From
394
pF ≥ 1.60 (experiment duration of ≥ 96 hours) the water retention of silt/gel mixtures
395
dropped down, which is especially significant for the higher gel incorporation rate of
396
0.7 wt %. In contrast, curve progressions of gel/sand mixtures at both rates remained
397
almost linear throughout the pF-range. Within the matrix potential range, the region
398
between pF 1.80 and 2.53 is of particularly interest for agronomic applications as a
399
control of irrigation.46 The amount of water in this pF region is thereby defined as field
400
capacity and represents the maximum of plant available water.46 The incorporation of
401
WS3-CA4% thus significantly improved the water retention of the sand at both rates
402
within this specific range (e.g. at pF 2.53: 0.2 or 0.7 wt % gel increased water retention by
403
70 or 300 %, respectively). However, the effect on the silty substrate at pF 2.53 in our
404
lab-scale survey was much smaller, with an increase of circa 12 or 20 %, by
405
incorporating 0.2 or 0.7 wt % dry gel granules, respectively.
406
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407 408
Figure 6. Water retention in soil substrates – water retained in a (A) sandy or (B) silty
409
soil or gel/soil mixtures as function of matrix potential (0 ≤ pF ≤ 2.53) and gel
410
incorporation rate (w/w soil). (hydrogel: WS3-CA4%; blue error bars: standard deviation,
411
n = 3)
412
This significant deviation between the two soil substrates thereby most probably
413
corresponds to both the different soil characteristics and the ionic character of the
414
lignocellulosic hydrogel. Cations – particularly of a divalent nature as Ca2+ – released
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from the soil substrate may have strongly impaired the swelling of the gel particles over
416
time by physically crosslinking the carboxylated polymer chains. This aspect can be
417
demonstrated by a very simple experiment, swelling WS3-CA4% in aqueous saline
418
solutions (SI, Figure S5). Accordingly, the addition of cations to the surrounding
419
medium strongly limited the water absorbency of the hydrogel under equilibrium
420
swelling conditions. Another reason for diminished swelling of WS3-CA4% in the silty
421
soil might have been its fine texture and, therefore, strengthened capillary forces within
422
soil pores. With the increasing matrix potential, the induced capillarity might thus have
423
entailed a drainage of the gel particles and an alignment of the water potential in gel
424
and surrounding soil substrate. Also, the soil pH (sand: 4.07, silt: 6.98) might have
425
affected the swelling properties of the gel. Especially, a soil pH below the pKA of
426
carboxylic groups (4.6)47 would cause, over time, a collapse of the gel network due to
427
the induced ion exchange. Nevertheless, throughout our short-time lab-scale
428
experiments, WS3-CA4% had a higher swelling capacity in the more acidic sandy
429
substrate.
430
According to our experiments, the incorporation of WS3-CA4% showed promising
431
potential for the sand within the examined pF range. However, to thoroughly illuminate
432
the effect of the gel on the water accessibility in soils, it would be essential to extent
433
water retention tests to pF = 4.2. Beyond a matrix potential of 4.2 (permanent wilting
434
point) water would be unavailable for plants.46,48
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Finally, a crucial point in further studies should be to translate the model-scale into
436
real applications, involving optimized synthetic upscaled procedures and field
437
experiments that offer realistic conditions. Also, the high loss of biomass due to
438
extraction/dissolution throughout the synthetic path (> 60 % for, e.g., WS3-CA4%)
439
demands for further process optimization and/or strategies to recover and valorize lost
440
material fraction. For the actual soil application, the structural integrity of the gel over
441
an adequate time period (e.g. one growth season) and its final biodegradation should be
442
central points of assessment. For the upscale to an economic volume, citric acid
443
crosslinking, as a very simple water-based, heat-induced reaction would allow for
444
continuous processing (e.g. on belt furnaces).
445 446
CONCLUSIONS
447
Herein we presented a simple synthetic approach to fabricate wheat straw-based
448
hydrogels starting from the unfractionated and carboxymethylated lignocellulosic
449
matrix. Citric acid was used as a cheap and non-toxic low-molecular-weight crosslinker.
450
Optimized reaction conditions led to mechanically stable hydrogels with moderate
451
swelling capacities of up to 50 gwater/g. Gel swelling and rheology were thereby
452
essentially determined by both the content of ionic carboxylate groups and the
453
crosslinking density. Moreover, DVS analysis revealed that the straw-based gels re-
454
swell after complete drying, implying a promising durability in terms of a potential soil
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application. Finally, incorporating one selected hydrogel into different model soil
456
substrates showed that already low gel rates (here: 0.2 wt %) may effectively increase
457
the water holding capacity of, especially, sandy soils.
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ASSOCIATED CONTENT
459
Supporting Information. A supporting information file (PDF) is available free of
460
charge containing: soil substrates´ characteristics; additional results for gel rheology,
461
crosslinking conditions vs. gel properties, DVS analysis, and swelling experiments.
462 463
AUTHOR INFORMATION
464
Corresponding Author
465
*
[email protected] 466
Present Addresses
467
Department
468
00076 Aalto, Espoo, Finland
469
Author Contributions
470
The manuscript was written through contributions of all authors. All authors have
471
given approval to the final version of the manuscript.
472
Funding Sources
473
The authors gratefully acknowledge the financial support from BASF SE (Germany) and
474
from the Graduate Academy, Technische Universität Dresden (Excellence Initiative of
475
the German federation and the federal states).
of Bioproducts and Biosystems, Aalto University, P.O. Box 16300, FIN-
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476
Notes
477
The authors declare no competing financial interests.
478
ACKNOWLEDGMENT
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479
The authors would like to thank Dr. Arndt Weiske (Institute of Plant and Wood
480
Chemistry, TU Dresden), Dr. Thomas Klinger and Gisela Ciesielske for ICP-OES
481
measurements and the assistance with water retention tests.
482
ABBREVIATIONS
483
Ad1(2), first (second) adsorption; CA, citric acid; De1, intermediate desorption; DI
484
water, deionized water; GAB, Guggenheim-Anderson-de Boer; ICP-OES, inductively
485
coupled plasma optical emission spectroscopy; IPA, isopropyl alcohol; RT, room
486
temperature; WS, wheat straw
487
REFERENCES
488
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For Table of Contents Use Only
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SYNOPSIS
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Herein, lignocellulosic biomass wastes – a sustainable polymer source – are used to
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fabricate hydrogels as miniature water reservoirs for agricultural applications.
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Figure 1. (A) Schematic representation of the synthetic path, and (B) ATR-IR spectra of (top to bottom) unmodified wheat straw, carboxymethylated straw (WS2) and (c) citric acid-crosslinked WS2 (16 wt % citric acid, 140 °C, 30 min). 272x208mm (300 x 300 DPI)
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Figure 2. Crosslinking of the carboxymethyl intermediates WS1, WS2 and WS3 – development of gel yield (◼), swelling ratio (○) and gel stiffness (◆, G´) as a function of the carboxymethyl group content. 160x94mm (300 x 300 DPI)
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Figure 3. Esterification of the carboxymethyl intermediate WS2 (2.56 mmol –CH2COONa g-1): gel yield (◼), swelling ratio in deionized water (○) and gel stiffness (◆, G´) as functions of (A) citric acid concentration, (B) reaction duration and (C) temperature. 160x279mm (300 x 300 DPI)
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Figure 4. Sorption isotherms of the first and second adsorption (Ad1/Ad2) and the intermediate desorption (De1) of gels based on WS1 (1.75 mmol –CH2COONa g-1) or WS3 (3.79 mmol –CH2COONa g-1). (crosslinking conditions: 140 °C, 30 min, 4 or 8 wt % citric acid) 175x80mm (300 x 300 DPI)
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Figure 5. Monolayer capacity M0 (blue bars/values) and water-accessible specific surface area AG (◻, black values) obtained by GAB model fitting of sorption isotherms of first and second adsorption (Ad1/Ad2) and intermediate desorption (De1). (Gels based on WS1 (1.75 mmol –CH2COONa g-1) or WS3 (3.79 mmol – CH2COONa g-1), crosslinking reaction: 140 °C, 30 min, 4 or 8 wt % citric acid) 119x70mm (300 x 300 DPI)
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Figure 6. Water retention in soil substrates – water retained in a (A) sandy or (B) silty soil or gel/soil mixtures as function of matrix potential (0 ≤ pF ≤ 2.53) and gel incorporation rate (w/w soil). (hydrogel: WS3-CA4%; blue error bars: standard deviation, n = 3) 144x209mm (300 x 300 DPI)
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