In this study we describe a novel high throughput, micro-macro approach for the identification and efficient design of biopolymer stabilised soil systems. At the 'microscopic' scale we propose a rapid Membrane Enabled Bio-Mineral Affinity Screening (MEBAS) approach supported by Mineral Binding Characterization (MBC) (TGA, ATR-FTIR and Zeta Potential), while at the 'macroscopic' scale, micro scale results are confirmed by Geotechnical Verification (GV) through unconfined compression testing. We illustrate the methodology using an exemplar mine tailings Fe2O3 -SiO2 system. Five different biopolymers were tested against Fe2O3: Locust Bean Gum, Guar Gum, Gellan Gum, Xanthan Gum and Sodium Carboxymethyl Cellulose. The screening revealed that Locust Bean Gum and Guar Gum have the highest affinity for Fe2O3, which was confirmed by MBC and GV. This affinity is attributed to the biopolymer's ability to form covalent C-O-Fe bonds through β-(1-4)-D-Mannan groups.Upon their 1% addition to a 'macroscopic' Fe2O3 based exemplar MT system, unconfined compressive strengths of 5171 kPa and 3848 kPa were obtained, significantly higher than for the other biopolymers and non-Fe systems. In the current study MEBAS gave an approximately 50-fold increase in rate of assessment compared to GV alone. Application of the proposed MEBAS -MBC -GV approach to a broad range of soil/earthwork components and additives is discussed.

Water scarcity in semi-arid/arid regions is driving the use of salt water in mining operations. A consequence of this shift, is the potentially unheeded effect upon Mine Tailing (MT) management. With existing stabilization/solidification methodologies exhibiting vulnerability to MT toxicity and salinity effects, it is essential to explore the scope for more environmentally durable sustainable alternatives under these conditions. Within this study we investigate the effects of salinity (NaCl, 0–2.5 M) and temperatures associated with arid regions (25 °C, 40 °C), on Locust Bean Gum (LB) biopolymer stabilization of MT exemplar and sand (control) soil systems. A cross-disciplinary ‘micro to macro’ pipeline is employed, from a Membrane Enabled Bio-mineral Affinity Screen (MEBAS), to Mineral Binding Characterisation (MBC), leading finally to Geotechnical Verification (GV). As predicted by higher Fe2O3 LB binding affinity in saline in the MEBAS studies, LB with 1.25 M NaCl, results in the greatest soil strength in the MT exemplar after 7 days of curing at 40 °C. Under these most challenging conditions for other soil strengthening systems, an overall UCS peak of 5033 kPa is achieved. MBC shows the critical and direct relationship between Fe2O3-LB in saltwater to be ‘high-affinity’ at the molecular level and ‘high-strength’ achieved at the geotechnical level. This is attributed to biopolymer binding group’s increased availability, with their ‘salting-in’ as NaCl concentrations rises to 1.25 M and then ‘salting-out’ at higher concentrations. This study highlights the potential of biopolymers as robust, sustainable, soil stabilization additives in challenging environments.

Sustainable biopolymer additives offer a promising soil stabilisation methodology, with a strong potential to be tuned to soil’s specific nature, allowing the tailoring of mechanical properties for a range of geotechnical applications. However, the biopolymer chemical characteristics driving soil mechanical property modifications have yet to be fully established. Within this study we employ a cross-scale approach, utilising the differing galactose:mannose (G:M) ratios of various Galactomannan biopolymers (Guar Gum G:M 1:2, Locust Bean Gum G:M 1:4, Cassia Gum G:M 1:5) to investigate the effect of microscale chemical functionality upon macroscale soil mechanical properties. Molecular weight effects are also investigated, utilising Carboxy Methyl Cellulose (CMC). Soil systems comprising of SiO2 (100%) (SiO2) and a Mine Tailing (MT) exemplar composed of SiO2 (90%) + Fe2O3 (10%) (SiO2 + Fe) are investigated. The critical importance of biopolymer additive chemical functionality for the resultant soil mechanical properties, is demonstrated..For Galactomannan G:M 1:5 stabilised soils the ‘high-affinity, high-strength’, mannose-Fe interactions at the microscale (confirmed by mineral binding characterisation) are attributed to the 297% increase in the SiO2 + Fe systems Unconfined Compressive Strength (UCS), relative to SiO2 only. Conversely for SiO2 Galactomannan-stabilised soils, when increasing the G:M ratio from 1:2 to 1:5, a 85% reduction in UCS is observed, attributed to mannose’s inability to interact with SiO2. UCS variations of up to a factor of 12 were observed across the biopolymer–soil mixes studied, in line with theoretically and experimentally expected values, due to the differences in the G:M ratios. The limited impact of molecular weight upon soil strength properties is also shown in CMC-stabilised soils. When considering a soil’s stiffness and energy absorbance, the importance of biopolymer–biopolymer interaction strength and quantity is discussed, further deciphering biopolymer characteristics driving soil property modifications. This study highlights the importance of biopolymer chemistry for biopolymer stabilisation studies, illustrating the use of simple low-cost, accessible chemistry-based instrumental tools and outlining key design principles for the tailoring of biopolymer–soil composites for specific geotechnical applications. Graphical abstract

Water scarcity in semi-arid/arid regions is driving the use of salt water in mining operations. A consequence of this shift, is the potentially unheeded effect upon Mine Tailing (MT) management. With existing stabilization/solidification methodologies exhibiting vulnerability to MT toxicity and salinity effects, it is essential to explore the scope for more environmentally durable sustainable alternatives under these conditions. Within this study we investigate the effects of salinity (NaCl, 0-2.5 M) and temperatures associated with arid regions (25 , 40), on Locust Bean Gum (LB) biopolymer stabilization of MT exemplar and sand (control) soil systems. A cross-disciplinary ‘micro to macro’ pipeline is employed, from a Membrane Enabled Bio-mineral Affinity Screen (MEBAS), to Mineral Binding Characterisation (MBC), leading finally to Geotechnical Verification (GV). As predicted by higher Fe2O3 LB binding affinity in the MEBAS studies, LB with 1.25M NaCl, results in the greatest soil strength in the MT exemplar after 7 days of curing at 40 °C. Under these most challenging conditions for other soil strengthening systems, an overall UCS peak of 5033 kPa is achieved. MBC shows the critical and direct relationship between Fe2O3 LB ‘high-affinity’ at the molecular level and ‘high-strength’ achieved at the geotechnical level. This is attributed to biopolymer binding group’s increased availability, with their ‘salting-in’ as NaCl concentrations rises to 1.25M and then ‘salting-out’ at higher concentrations. This study highlights the potential of biopolymers as robust, sustainable, soil stabilization additives in challenging environments.

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