Physical property-based screening of polyurethane foam for gold enrichment from rock gold ores

Abstract

To address the technical challenges of significant variations in gold extraction efficiency and the absence of a scientific screening basis for commercially available unmodified polyurethane foam (U-PUF) in gold enrichment from rock gold ores, this study systematically investigated the correlations between six physical properties of 18 U-PUF samples and the gold extraction efficiency in accordance with General flexible polyurethane cellular plastics (GB/T 10802-2023). The gold enrichment mechanism was elucidated by combining adsorption kinetics, competitive adsorption characteristics, and microscopic characterization techniques. The results demonstrated that the optimal physical parameters of U-PUF with a gold adsorption rate of ≥95% were determined as follows: white color, density of 30–40 kg m⁻³, 25% indentation hardness of 105–140 N, 40% indentation hardness of 130–150 N, rebound rate of 50–65%, and pore size of 100–400 µm (fine pore). This type of U-PUF exhibited an optimal adsorption capacity of 3.91 mg g⁻¹, and its adsorption process followed the three-stage intraparticle diffusion model, reaching adsorption equilibrium within 30 min with excellent homogeneous adsorption performance. FTIR analysis confirmed that no chemical bonding was involved in the gold adsorption by U-PUF, while SEM-EDS characterization revealed that the gold existed as multi-scale aggregates with sizes ranging from 300 nm to 15 µm on the porous framework of U-PUF. The enrichment mechanism was clarified as follows: in an acidic system, the secondary amino groups on U-PUF were protonated, which then captured [AuCl₄]⁻ anions via electrostatic interaction and subsequently reduced them to elemental gold in situ. The U-PUF screened by the optimal parameters showed good consistency between the determined values and the certified values for 26 gold ore reference materials, and the determined values for actual rock gold ore samples were not significantly different from those obtained by the activated carbon enrichment method and fire assay. The screening standard for U-PUF physical properties established in this study resolves the dilemma of material selection in gold enrichment for batch geological samples, and provides theoretical support and technical specifications for the standardized application of U-PUF.

To address significant variations in gold extraction efficiency and the absence of screening criteria for unmodified polyurethane foam (U-PUF), correlations between six physical properties of 18 U-PUF samples and gold recovery were systematically investigated per GB/T 10802-2023.

1. Introduction

Gold (Au), as a critical precious metal with industrial, medical and financial properties, occupies an irreplaceable position in electronic manufacturing, catalytic materials, precision instruments and the global reserve system.^1,2 With the rapid development of emerging industries (e.g., semiconductors and new energy), the global demand for gold has been continuously growing. However, its average abundance in the earth's crust is merely 4 ng g⁻¹ (approximately 0.004 g t⁻¹), making it a typical ultra-low abundance element.³ Although the detection limits of modern analytical techniques such as inductively coupled plasma mass spectrometry (ICP-MS) and graphite furnace atomic absorption spectrometry (GFAAS) have reached the ng g⁻¹ or even pg g⁻¹ level,⁴ the massive coexisting matrix elements (e.g., Fe, Al and Cu) in rock gold ore samples cause severe spectral interference and matrix effects. Meanwhile, gold exists in complex occurrence forms (e.g., micro-inclusions and sulfide-bound forms), which results in it being difficult for the accuracy and precision of direct determination to meet the requirements of geological exploration and resource evaluation.^5,6 Therefore, efficient enrichment and separation remain a core prerequisite step for the accurate determination of gold in rock gold ore samples.

At present, the enrichment and separation technologies for Au mainly include fire assay,⁷ coprecipitation,⁸ solvent extraction,⁹ and solid phase extraction (SPE). Among these techniques, SPE has become the preferred method for the rapid pretreatment of Au in large batches of geological samples due to its advantages of simple operation, high enrichment factor, low organic solvent consumption, easy coupling with automation, and controllable cost.^10,11 As one of the most widely used adsorbent materials in the SPE system, polyurethane foam (PUF) has been extensively adopted both domestically and internationally by virtue of its porous structure, high specific surface area, low cost, and ease of storage and handling.¹² The application of PUF in the field of adsorption and separation can be traced back to the pioneering work of Bowen, who first confirmed the adsorption performance of PUF for inorganic ions and organic compounds in aqueous solutions.¹¹ Subsequently, Braun et al.¹⁰ realized the selective separation and recovery of gold by PUF through the optimization of hydrochloric acid medium system, which laid the foundation for its application in the pretreatment of geological samples.

In recent years, research on modified polyurethane foam has become a research hotspot. The grafting of functional groups such as amino and mercapto groups or composite modification with nanoparticles can significantly improve the adsorption capacity and selectivity for Au.^13,14 However, such modified materials have inherent drawbacks including a substantial increase in production costs, complex preparation processes (requiring special radiation or catalytic equipment), poor batch stability, and great difficulties in large-scale application.^15,16 In contrast, U-PUF has been incorporated into a number of standard methods due to its simple preparation process, low cost and stable performance, such as Methods for Analysis of Regional Geochemical Samples – Part 35: Determination of Gold Content – Foam Enrichment-Flame Atomic Fluorescence Spectrometry (DZ/T 0279.35-2022),¹⁷ Methods for Analysis of Regional Geochemical Samples – Part 4: Determination of Gold Content – Foam Enrichment-Inductively Coupled Plasma Mass Spectrometry (DZ/T 0279.4-2016),¹⁸ and Methods for Analysis of Regional Geochemical Samples – Part 19: Determination of Gold Content – Foam Enrichment-Graphite Furnace Atomic Absorption Spectrometry (DZ/T 0279.19-2016).¹⁹ These standards explicitly specify that commercially available U-PUF is sufficient to meet the requirements for Au enrichment in geological samples.

However, in practical applications, it has been found that the gold enrichment efficiency of U-PUF produced by different manufacturers for rock gold ore samples varies significantly. Furthermore, existing studies have not clarified the correlation between the physical properties of U-PUF and its Au enrichment efficiency,^20,21 resulting in the lack of a scientific basis for U-PUF screening in batch sample analysis. This has become a key bottleneck restricting analytical efficiency and data consistency, and has also limited the popularization and application of U-PUF in high-precision geological exploration.²² General Flexible Polyurethane Foam (GB/T 10802-2023)²³ has clearly specified the key physical property indexes of U-PUF such as density, hardness, rebound rate and pore size, but the corresponding relationship between these indexes and Au adsorption performance has not yet been established.

Accordingly, in accordance with the national standard GB/T 10802-2023, this study systematically investigated the Au enrichment efficiency of commercially available U-PUF with various physical property parameters (density, hardness, pore size distribution, rebound rate, etc.) from rock gold ore samples. This study aims to establish a U-PUF screening method based on conventional physical property indexes and solve the technical problem of material selection in batch sample analysis. Meanwhile, combined with adsorption kinetics and characterization analyses, the adsorption characteristics and enrichment mechanism of U-PUF for Au were preliminarily revealed, which provides theoretical support and technical reference for the standardized application of U-PUF in Au enrichment from geological samples.

2. Experimental section

2.1. Reagents and materials

All chemical reagents used in this study, including hydrochloric acid (HCl), nitric acid (HNO₃), ferric chloride, ammonium fluoride and thiourea, were of analytical grade. Ultrapure deionized water with a resistivity of 18.2 MΩ·cm at 25 °C was employed throughout the experiments. The main freshly prepared solutions were 250 g L⁻¹ ferric chloride solution, 20% ammonium fluoride solution, 10 g L⁻¹ thiourea solution and aqua regia. The Au single-element standard solution (GSB 04-1715-2004, 1000 µg mL⁻¹), dissolved in 1.5 mol L⁻¹ HCl, was purchased from the National Analysis Center for Nonferrous Metals and Electronic Materials. The working standard solutions of Au were prepared by the serial dilution method. 18 commercial unmodified polyurethane foam (U-PUF) samples were obtained from Chengdu Jinjiang Foam Co., Ltd, and their detailed physical property indexes are presented in Table 1.

Table 1. Physical property indexes of 18 commercial U-PUF samples.

No.	Color	Density (kg m−3)	25% Indentation hardness (N)	40% Indentation hardness (N)	Rebound rate (%)	Pore size
1	White	17	85	110	38	400–1000 µm (coarse pore)
2	White	30	110	135	54	100–400 µm (fine pore)
3	White	45	80	110	74	400–1000 µm (coarse pore)
4	White	40	105	130	64	100–400 µm (fine pore)
5	White	30	200	260	38	100–400 µm (fine pore)
6	Yellow	13	55	75	40	100–400 µm (fine pore)
7	White	15	60	80	41	400–1000 µm (coarse pore)
8	Orange	29	80	105	60	600–1200 µm (ultra-coarse pore)
9	Citrus white	38	100	135	68	400–1000 µm (coarse pore)
10	White	15	60	80	41	600–1200 µm (ultra-coarse pore)
11	Yellow	12	50	80	37	400–1000 µm (coarse pore)
12	Yellow	12	60	90	36	400–1000 µm (coarse pore)
13	White	32	115	160	55	600–1200 µm (ultra-coarse pore)
14	White	20.5	35	50	50	100–400 µm (fine pore)
15	White	38	125	155	59	100–400 µm (fine pore)
16	White	35	140	175	54	100–400 µm (fine pore)
17	Purple	30	150	185	40	100–400 µm (fine pore)
18	White	33.5	≥350	≥350	≥350	100–400 µm (fine pore)

2.2. Apparatus and instruments

Sample Pretreatment Equipment: Electronic balance (JJ500, Changshu Shuangjie Testing Instrument Co., Ltd), muffle furnace (SX-10-12PII, Test Instrument Co., Ltd) and self-made closed sample-dissolving water bath with a cover. Adsorption and Desorption Equipment: Reciprocating oscillator (DC-400, Chengdu Haiguang Instrument Co., Ltd) and constant-temperature water bath (SY-100, Chengdu Haiguang Instrument Co., Ltd). Detection and Characterization Instruments: Flame atomic absorption spectrometer (GG-910, Beijing Haiguang Instrument Co., Ltd), scanning electron microscope (SEM) imaging was performed on an S-4800 scanning electron microscope (Hitachi, Japan), fourier transform infrared (FTIR) spectra were recorded using a Nicolet iS5 FTIR spectrometer (Thermo Scientific, USA) and microscopic imaging was conducted on an LV100NPOL.

2.3. Experimental methods

2.3.1. Pretreatment of U-PUF

The U-PUF was cut into cuboids with dimensions of 3 × 2 × 1 cm (approximately 0.2 g per piece). The U-PUF blocks were soaked in 10% (volume fraction, v/v) HCl for 6 h to remove weakly alkaline impurities and residual additives on the surface. Subsequently, the U-PUF was washed with ultrapure water until neutral (pH 6.5–7.5), squeezed to remove excess water, air-dried, and then sealed for subsequent use.

2.3.2. Sample digestion

Accurately 20.0 g of rock gold ore sample (weighed to the nearest 0.1 g) was placed into a 50 mL porcelain crucible. The crucible was transferred into a muffle furnace, and the temperature was raised from room temperature to 700 °C by programmed heating. The sample was roasted at 700 °C for 2 h to eliminate sulfides and organic matter. After cooling to room temperature, the roasted sample was transferred into a 250 mL polyethylene plastic bottle. Then, 50 mL of aqua regia (1 + 1), 1 mL of 250 g L⁻¹ ferric chloride solution, and 5 mL of 20% ammonium fluoride solution were added sequentially. The bottle was capped and heated in a water bath at 100 °C for 120 min. After cooling to room temperature, the digest solution was prepared for subsequent experiments.

2.3.3. Enrichment and desorption of Au

100 mL of ultrapure water was added into the above digest solution, followed by one piece of pretreated U-PUF. The mixture was oscillated on a reciprocating oscillator for 60 min to complete the adsorption process. The U-PUF was taken out, rinsed with tap water to remove residual acid and ore residues on the surface, and then washed with deionized water. After squeezing out excess water, the U-PUF was placed into a 25 mL colorimetric tube containing 20 mL of 10 g L⁻¹ thiourea solution. The desorption process was conducted in a boiling water bath for 30 min. The U-PUF was quickly pressed and removed, and the desorption solution was cooled to room temperature. The Au content in the desorption solution was determined by atomic absorption spectrometry (AAS). The Au adsorption rate (E) of U-PUF was calculated using the following equation:

E = C₁/C₀ × 100% 1

where C₀ and C₁ represent the initial concentration of Au in the solution and the adsorbed concentration of Au on U-PUF, respectively.

2.4. Experimental methods (continued)

2.4.1. Establishment of calibration curve

0 mL, 0.50 mL, 1.00 mL, 2.00 mL, 4.00 mL and 6.00 mL of 10 µg mL⁻¹ gold standard working solution were transferred into 250 mL polyethylene plastic bottles, respectively. Then, 50 mL of aqua regia, 1 mL of ferric chloride solution, 100 mL of ultrapure water and one piece of pretreated U-PUF were added sequentially. The subsequent procedures were identical to those described in Section 2.3.3. The calibration curve was plotted with the gold concentration as the abscissa and the corresponding absorbance as the ordinate.

2.4.2. Adsorption kinetics and competitive adsorption experiments

Adsorption kinetics experiment: 12 pieces of U-PUF No. 2 (high-efficiency adsorption) and 12 pieces of U-PUF No. 13 (low-efficiency adsorption), with an accurate mass of 0.2 g per piece, were weighed and placed into standard solutions containing 40 µg of Au, respectively. The oscillation adsorption time was set as 0, 1, 2, 3, 4, 5, 10, 15, 20, 30, 40 and 60 min. The other operations were consistent with Section 2.3.3. The gold adsorption rates at different time intervals were calculated, and the kinetic models were fitted accordingly.

Competitive adsorption experiment: U-PUF No. 2 and No. 13 were accurately weighed at masses of 0.05, 0.1, 0.2, 0.3 and 0.4 g, respectively. Two groups of experiments were designed: ① 5 pieces of U-PUF with the same type but different masses were separately placed into solutions containing 200 µg of Au for adsorption; ② 10 pieces of U-PUF with different types and masses (5 pieces of No. 2 and 5 pieces of No. 13) were placed together into a solution containing 200 µg of Au for adsorption. The Au adsorption capacity per unit mass of U-PUF (K) and the relative standard deviation (RSD) were calculated.

2.5. Method validation

8 types of U-PUF (Y-1–Y-8) conforming to the optimal physical parameters (obtained from 5 different manufacturers) were selected for the adsorption-desorption validation (n = 2) in standard solutions containing 40 µg of Au. U-PUF No. 2 and No. Y-4 were adopted to determine 26 gold ore certified reference materials, and the consistency between the determined values and the certified values was compared. Four actual rock gold ore samples were selected for parallel determinations (n = 4) using U-PUF No. 2, No. Y-4, No. Y-8, activated carbon enrichment method and fire assay method respectively, to verify the reliability of the established method. The fire assay test was completed by the Institute of Multipurpose Utilization of Mineral Resources, Chinese Academy of Geological Sciences.

3. Results and discussion

3.1. Adsorption differences of Au by U-PUF with different physical properties

The 18 types of U-PUF were pretreated via three distinct methods, namely no treatment, boiling in water for 2 h and soaking in 10% HCl for 6 h, respectively. Subsequently, the treated U-PUF samples were placed into polyethylene plastic bottles containing 30 mL of Au standard solution (4 µg g⁻¹). The enrichment and desorption processes were carried out in strict accordance with the procedures described in Section 2.3.3. Triplicate parallel experiments were performed for the U-PUF samples under each pretreatment condition. The gold adsorption rates of the 18 U-PUF samples obtained through the three pretreatment methods are illustrated in Fig. 1.

Fig. 1. Au adsorption rates of U-PUF with different physical property indexes (note: error bars represent the standard deviations of triplicate parallel experiments, n = 3; (a) PUF color, (b) PUF density; (c) PUF indentation hardness (25%), (d) PUF indentation hardness (40%), (e) PUF rebound, (f) PUF PORE size).

U-PUF generally contains weakly alkaline ions and impurities, and thus pretreatment is required prior to use. The results showed that the Au adsorption rates of the same U-PUF after water boiling or HCl soaking were significantly higher than that of the untreated one, and the adsorption rate after acid soaking was superior to that after water boiling. Therefore, the acid soaking method was adopted for U-PUF pretreatment in this study.

In previous studies, little attention has been paid to the effects of physical and chemical properties on the Au extraction efficiency of PUF.^10,12,13,24 Most studies directly adopted unmodified PUF for gold extraction,^{4,6,7,11,25,26} or focused on the Au extraction efficiency of PUF after loading modification.^9,12,13,15 The experimental results (Fig. 1) indicated that PUF with different physical property indexes exhibited significant differences in Au extraction rates. In terms of color, only part of the white PUF achieved a Au extraction rate of 95% and above, while those of PUF with other colors were all lower than 95%, while the specific chemical identity of color additives was not characterized in this study, their presence likely reduces the density of accessible protonated amine groups, as suggested by the consistently lower adsorption rates observed in colored foams.²⁶ For density, the main densities of PUF with a Au extraction rate ≥95% were 30–40 kg m⁻³ and 20.5 kg m⁻³. For 25% indentation hardness, PUF with values of 105–140 N and 35 N showed a Au extraction rate of 95% and above. For 40% indentation hardness, PUF with values of 130–150 N and 50 N achieved a Au extraction rate of 95% and above. For rebound rate, PUF with a rebound rate of 50–65% exhibited a gold extraction rate of 95% and above. For pore size, only part of the fine-pore PUF reached a Au extraction rate of 95% and above, while those of coarse-pore and ultra-coarse-pore PUF were all lower than 95%.

In summary, the optimal physical property parameters of U-PUF for Au enrichment were determined as follows: white color, density of 30–40 kg m⁻³, 25% indentation hardness of 105–140 N, 40% indentation hardness of 130–150 N, rebound rate of 50–65%, and pore size of 100–400 µm (fine pore). The correlation between physical properties and adsorption performance can be rationalized as follows: the fine pore structure (100–400 µm) provides higher specific surface area and uniform diffusion pathways for [AuCl₄]⁻, whereas coarse pores induce channeling effects;^10,13 moderate density (30–40 kg m⁻³) and indentation hardness ensure sufficient mechanical stability without hindering ion accessibility to protonated secondary amine groups;^9,26 the rebound rate (50–65%) reflects optimal viscoelasticity for continuous site renewal during oscillation;⁹ white color indicates the absence of pigment additives that might occupy active sites.²⁶

3.2. Adsorption capacity characteristics

U-PUF No. 2 and No. 13 were selected as representative high- and low-efficiency samples, respectively. Notably, No. 13 possesses similar density and hardness to No. 2 but exhibits ultra-coarse pores (600–1200 µm vs. 100–400 µm), making it an ideal control to isolate the specific impact of pore morphology on adsorption behavior.^21,22 Maximum adsorption capacity refers to the maximum amount of Au that can be adsorbed by PUF, reflecting the ultimate adsorption capacity of PUF under ideal conditions. Optimal adsorption capacity is defined as the adsorption capacity when PUF achieves the highest Au adsorption efficiency. It is generally lower than the maximum adsorption capacity but more representative of the effective adsorption capacity in practical applications.

The comparison results of adsorption capacities between U-PUF No. 2 (high-efficiency) and No. 13 (low-efficiency) are presented in Fig. 2. ① Maximum adsorption capacity: No significant difference was observed between the two samples (12.5 mg g⁻¹ for No. 2 and 12.4 mg g⁻¹ for No. 13), indicating that the ultimate adsorption capacity of U-PUF is mainly determined by the bulk structure of the material. ② Optimal adsorption capacity: U-PUF No. 2 maintained an adsorption rate higher than 95% when the Au content was ≤800 µg, with an optimal adsorption capacity of 3.91 mg g⁻¹. In contrast, the adsorption rate of U-PUF No. 13 was consistently lower than 90%, showing no definite optimal adsorption range. This discrepancy originates from the difference in the number and distribution uniformity of active sites on the U-PUF surface. The high-efficiency U-PUF possesses a higher density of secondary amine groups (approximately 0.85 mmol g⁻¹) with a more homogeneous distribution, which provides sufficient adsorption sites for gold ions.²⁶ U-PUF is intended for single-use disposable application in batch gold analysis, as the harsh acidic conditions compromise structural stability upon regeneration attempts.

Fig. 2. Relationship between adsorption capacity and Au content of U-PUF No. 2 (a) and No. 13 (b).

3.3. Competitive adsorption characteristics

The experimental data of Group ① (Fig. 3a) showed that the total amount of Au adsorbed by U-PUF No. 2 was 202.3 µg, with a total adsorption rate of 101.2%, while that by U-PUF No. 13 was 180.3 µg, with a total adsorption rate of 90.2%. The average K value of No. 2 (189.3) was higher than that of No. 13 (174.5), and the RSD of K for No. 2 (2.35%) was significantly lower than that for No. 13 (8.98%). The Au adsorption capacity of No. 2 U-PUF was stronger than that of No. 13 U-PUF, indicating that No. 2 U-PUF exhibited superior homogeneous adsorption characteristics for Au. The experimental data of Group ② (Fig. 3b) showed that the total amount of Au adsorbed by the 10 pieces of U-PUF was 194.0 µg, with a total adsorption rate of 97.0%. Specifically, the 5 pieces of No. 2 U-PUF adsorbed 124.0 µg of Au in total (total adsorption rate: 62.0%), whereas the 5 pieces of No.13 U-PUF adsorbed 70.1 µg of Au in total (total adsorption rate: 35.0%). The average K value of No.2 U-PUF (124.0) was significantly higher than that of No. 13 U-PUF (69.7), confirming that the Au adsorption capacity of No. 2 U-PUF was remarkably superior to that of No. 13 U-PUF. It can be seen that there are significant differences in the competitive adsorption characteristics of Au among different commercial U-PUF products. The U-PUF with excellent adsorption capacity presents favorable homogeneous adsorption properties for Au, and the high-efficiency U-PUF shows stronger adsorption selectivity towards [AuCl₄]⁻ anions, which is directly related to the number of protonated active sites on the U-PUF surface.²⁷

Fig. 3. Competitive adsorption results of Au by different U-PUF ((a) Group 1, (b) Group 2).

3.4. Adsorption kinetics characteristics

The adsorption of Au by PUF occurs mainly due to the unsaturated force field of interfacial molecules on the solid surface, i.e., the presence of a solid surface force. This force can adsorb molecules, atoms or ions from the external environment and form a monomolecular layer or multimolecular layer on the adsorption surface.⁹ Contact time is a critical factor influencing the adsorption of Au by PUF.^10,13,16

The adsorption curves (Fig. 4) show that the Au ion extraction rates of the two U-PUF samples were the fastest within the first 5 min, which might be attributed to the fact that the number of adsorption sites in the initial stage was more abundant than that in the subsequent stages. Therefore, Au ions could interact with these sites more easily, resulting in a high adsorption rate. Once the adsorption sites on the adsorbent surface were saturated, the adsorption rate was controlled by the mass transfer rate from the exterior of the adsorbent particles to the internal active sites.¹⁶ In addition, the high concentration gradient in the initial stage also contributed to the rapid adsorption rate. The Au adsorption rate increased relatively slowly from 5 to 20 min, and this slow adsorption rate was ascribed to the slow diffusion of solutes into the interior of the adsorbent.²¹ With the prolongation of contact time, the amount of adsorbed Au ions increased gradually. The adsorption became extremely slow and tended to stabilize after 20 min, and adsorption equilibrium was achieved at 30 min. After reaching equilibrium, the adsorption rate of PUF No. 2 was above 95%, while that of PUF No. 13 was below 90%.

Fig. 4. Effect of contact time on Au adsorption by different U-PUF.

During the oscillatory adsorption process, Au ions were transferred from the liquid phase to the PUF surface and further diffused into the internal pores. This process can be explained by the intraparticle diffusion kinetic model, expressed as: Q_t = k_i × t_1/2, where k_i is the intraparticle diffusion rate constant (mg·g⁻¹·min^−0.5) and t is the oscillation time (min). As shown in Fig. 5, the low correlation coefficients (R² = 0.73 for U-PUF No. 2 and R² = 0.55 for U-PUF No. 13) indicate that the overall adsorption reaction was not linear but multi-linear, suggesting that two or more steps occurred during the adsorption process.²² The adsorption curve was not linear across the entire time range but could be divided into three linear regions, and the correlation coefficients of the regional fitting were significantly increased, confirming that the adsorption involved multiple stages. In the inset of Fig. 5, the linear relationship between the adsorption capacity and the square root of time presented three linear segments, indicating that the adsorption of Au ions on U-PUF underwent three steps. Firstly, rapid adsorption occurred within the initial 5 min, which was attributed to external surface adsorption, where the adsorbate diffused from the bulk solution to the external surface of the adsorbent or through boundary layer diffusion. Secondly, a gradual adsorption stage took place, during which the intraparticle diffusion rate was the rate-controlling step. Thirdly, the adsorption became very slow and tended to stabilize, approaching the equilibrium stage and the maximum adsorption capacity.^16,28

Fig. 5. Intraparticle diffusion kinetic model of Au ions on PUF ((a): PUF-2; (b): PUF-13).

3.5. Microscopic morphology and structural characteristics

3.5.1. Optical microscopy observation

Blank and gold-adsorbed U-PUF No. 2 and No. 13 were cut into small slices with a size of 1–2 mm, and their surface morphological characteristics were observed under an optical microscope, as presented in Fig. 6. The pores of U-PUF No. 2 (Fig. 6a and b) were uniformly distributed with relatively consistent pore sizes. In contrast, U-PUF No. 13 (Fig. 6c and d) displayed irregular pore sizes and heterogeneous pore morphologies. Compared with the blank U-PUF (Fig. 6a and c), the U-PUF after Au enrichment (Fig. 6b and d) only exhibited a color change, which was mainly caused by the corrosion of aqua regia during the Au enrichment process. No significant alterations were found in the pore morphology and pore size. Unfortunately, the morphological features of the enriched Au on U-PUF were not detected under the optical microscope.

Fig. 6. Microscopic morphological characteristics of PUF No. 2 and No. 13 ((a) blank U-PUF No. 2, (b) Au-enriched U-PUF No. 2, (c) blank U-PUF No. 13, (d) Au-enriched U-PUF No. 13).

3.5.2. FTIR analysis

Fig. 7 presents the Fourier transform infrared (FTIR) spectra of U-PUF No. 2 in the untreated, acid-soaked and Au-enriched states. The spectral profiles of U-PUF under the three treatment conditions are basically consistent in the full wavenumber range of 4000–400 cm⁻¹. No peak shift, peak intensity variation or new characteristic peak generation was observed for the characteristic absorption peaks of amino groups (3300–3400 cm⁻¹) and carbonyl groups (1700–1730 cm⁻¹) in the U-PUF framework after Au enrichment, indicating that the chemical structure of U-PUF remained intact during the Au adsorption process. FTIR spectroscopy can effectively differentiate physical adsorption from chemical adsorption according to the changes in vibration signals of functional groups. Chemical adsorption would cause significant shift or disappearance of characteristic peaks owing to the formation of new chemical bonds between the adsorbent and adsorbate. In contrast, physical adsorption merely depends on non-covalent interactions including van der Waals forces and electrostatic interactions, which will not modify the intrinsic functional group structure of the adsorbent. In this study, the active sites (e.g., amino and carbonyl groups) on U-PUF did not form chemical bonds with Au, confirming that the adsorption of Au by U-PUF does not belong to chemical adsorption, and no chemical modification of functional groups is involved in the adsorption process.^27,29

Fig. 7. FTIR spectra of U-PUF.

3.5.3. SEM-EDS characterization

SEM was employed to perform microscopic characterization on the gold-adsorbed U-PUF, revealing the structural characteristics of the U-PUF matrix and the occurrence morphology of Au at different magnifications (Fig. 8).

Fig. 8. Structural characteristics of U-PUF and morphological characteristics of enriched Au under SEM ((a): U-PUF structural; (b): 350× magnification; (c): 1000× magnification; (d): 2000× magnification; (e): 10000× magnification; (f): 60 000×magnification; (g): 9000× magnification; (h): EDS point scanning).

Fig. 8a displays the SEM image at 60× magnification. The results indicate that U-PUF presents a typical porous three-dimensional spherical network structure, with a smooth and continuous network skeleton, and the pore size is concentrated at approximately 200 µm. This structure provides sufficient specific surface area and spatial sites for the diffusion, adsorption and enrichment of Au ions, which serves as the structural basis for the superior Au adsorption performance of PUF.^9,10

With the increase of magnification, the occurrence morphology of Au on the U-PUF surface becomes gradually distinct.¹³ At 350× magnification (Fig. 8b), obvious bright white spots can be observed on the U-PUF skeleton surface, indicating the initial local aggregation of Au element on the PUF surface. At 1000× magnification (Fig. 8c), Au microparticles in the form of aggregates are clearly visible with a particle size of approximately 5 µm, confirming that Au completes the nucleation and growth process on the active sites of PUF. At 2000× magnification (Fig. 8d), the Au aggregates exhibit an irregular polyhedral morphology with sharp edges and corners as well as flat crystal planes/fracture surfaces, showing typical characteristics of equiaxed brittle particles,³⁰ with a particle size ranging from 10 to 15 µm. At 10000× magnification (Fig. 8e), the Au aggregates appear as bright white particles with an irregular butterfly/dumbbell shape, which are composed of two approximately polygonal bulk regions connected by a narrow neck. The particle size is reduced to 3–4 µm, reflecting the morphological evolution of Au aggregates. At a high magnification of 60 000× (Fig. 8f), the Au aggregates are further refined into nanoparticles, presenting an irregular cluster morphology. They are agglomerated by multiple fine subunits with a rounded and wrinkled surface profile, and the lateral dimension is about 300–400 nm. At 9000× magnification (Fig. 8g), the gold-enriched phases are distributed as irregular angular and clastic aggregates, directly reflecting the local enrichment characteristics of gold element on the PUF surface. To identify the chemical composition of the bright white regions, EDS point scanning analysis was conducted (Fig. 8h). The results show a prominent characteristic peak of Au element, confirming that this region is the gold-enriched phase and further verifying the SEM observation results.

Overall, the porous three-dimensional spherical network structure of U-PUF provides an ideal spatial environment and active sites for the evolution process of “adsorption – nucleation – growth – aggregation” of Au on its surface.^9,10 Au exists on the U-PUF surface in the form of multi-scale aggregates, undergoes selective local enrichment to form irregular angular and clastic aggregates, and finally occurs as irregular independent particles/aggregates at the active sites of U-PUF, demonstrating the high-efficiency adsorption and enrichment capacity of U-PUF for Au.^13,30

3.6. Enrichment mechanism

According to the FTIR analysis, no obvious shift was observed in the characteristic peaks of U-PUF after gold adsorption, which can exclude the possibility of coordinate bonding between N/O atoms and Au(iii). Combined with the SEM observation results, Au exists in the form of multi-scale aggregates at the active site regions of the PUF skeleton.

The comprehensive experimental results indicate that the enrichment process of Au(iii) by U-PUF is dominated by protonated electrostatic adsorption accompanied by in situ reduction and deposition, rather than conventional physical adsorption or coordinate chemical adsorption. In the carbamate structure (−O–CO–NH−) of the U-PUF molecular chain, the secondary amine group (−NH−) serves as the key protonated active site.²⁹ In the acidic HCl system, Au(iii) exists as the stable tetrachloroaurate(iii) anion [AuCl₄]^-.⁹ After protonation of the secondary amine groups on the PUF skeleton, [AuCl₄]⁻ is captured via electrostatic interaction, and Au(iii) is further reduced to elemental gold. Van der Waals forces and hydrogen bonds only act as minor auxiliary forces.^13,31Protonation of secondary amine groups: PUF-NH⁻ + H⁺ → PUF-NH₂⁺Electrostatic adsorption and capture of Au anions: PUF-NH₂⁺ + [AuCl₄]⁻ → PUF-NH₂⁺···[AuCl₄]⁻In situ reduction: [AuCl₄]⁻ + 3e⁻→ Au(0)↓ + 4Cl⁻

3.7. Method validation

3.7.1. Validation of U-PUF from different manufacturers

According to the recommended indexes, 8 types of U-PUF (Y-1 to Y-8) conforming to the optimal parameters were selected from 5 U-PUF manufacturers for gold adsorption experiments (n = 2). The results are presented in Table 2. The results showed that among the 8 types of U-PUF, only the average adsorption rates (n = 2) of Y-4 and Y-5 were 94.8% ± 0.81% and 94.7% ± 0.46%, respectively, which were slightly lower than 95%. The average adsorption rates of the other 6 foam samples were all higher than 95%. The Au adsorption rates of U-PUF selected based on the physical parameters determined in this study were basically around 95%, which can well meet the requirements for Au enrichment.

Table 2. Au adsorption rates of 8 types of PUF selected according to the optimal recommended indexes.

No.	Product name	Color	Density (kg m−3)	25% Indentation hardness (N)	40% Indentation hardness (N)	Rebound rate (%)	Pore size	Adsorption rate (%)	Remarks
Y-1	Zhong 52	White	35	120	150	55	Fine pore	99.3 ± 0.65	Company 1
Y-2	151	White	33	125	155	57	Fine pore	96.5 ± 1.29	Company 1
Y-3	651	White	35	120	145	57	Fine pore	96.6 ± 0.13	Company 1
Y-4	D22	White	36	110	140	55	Fine pore	94.8 ± 0.81	Company 2
Y-5	Zhong 33	White	38	105	130	52	Fine pore	94.7 ± 0.46	Company 2
Y-6	No. 34	White	35	130	150	58	Fine pore	97.4 ± 0.13	Company 3
Y-7	Q6	White	30	125	145	60	Fine pore	96.0 ± 0.25	Company 4
Y-8	42	White	32	135	150	53	Fine pore	97.5 ± 0.75	Company 5

3.7.2. Validation of certified reference materials and actual samples

Au adsorption tests were performed on 26 gold ore certified reference materials (CRMs) using U-PUF No. 2 and No. Y-4, and the results are illustrated in Fig. 9. The determined values obtained by the two types of U-PUF were basically consistent with the certified values of the reference materials.

Fig. 9. Determination results of Au content in 26 gold ore certified reference materials by U-PUF No. 2 and No. Y-4.

Four common rock gold ore samples (No. 1#, 2#, 3# and 4#) were selected for enrichment tests using U-PUF (No. 2, No. Y-4 and No. Y-8), activated carbon and fire assay (n = 4). A comparison of the test results (Table 3) shows that the determination results of U-PUF No. 2 were consistent with those of the other two U-PUFs (No. Y-4 and No. Y-8) conforming to the recommended physical indexes. The results of the three U-PUFs were also consistent with those obtained by the activated carbon method and the fire assay method.

Table 3. Test results of common rock gold ore samples (g t⁻¹) (n = 4).

Sample no.	U-PUF No. 2	U-PUF No. Y-4	U-PUF No. Y-8	Activated carbon	Fire assay
1#	0.91 ± 0.035	0.89 ± 0.015	0.91 ± 0.020	0.90 ± 0.028	0.97 ± 0.013
2#	1.82 ± 0.030	1.75 ± 0.025	1.80 ± 0.035	1.84 ± 0.53	1.74 ± 0.038
3#	2.91 ± 0.019	2.84 ± 0.040	2.82 ± 0.05	2.84 ± 0.005	2.81 ± 0.11
4#	5.45 ± 0.083	5.30 ± 0.042	5.36 ± 0.12	5.33 ± 0.10	5.40 ± 0.21

4. Conclusion

Aiming at the problems of the lack of screening criteria and large discrepancies in enrichment efficiency when commercial U-PUF is applied to Au extraction from rock gold ores, the correlation between the physical properties and gold enrichment efficiency of 18 commercial U-PUF products was systematically studied in accordance with GB/T 10802-2023. The core conclusions were verified and clarified by a variety of characterization methods as follows: the high-performance U-PUF for Au enrichment shall meet the optimal physical parameters: white color, density of 30–40 kg m⁻³, 25% indentation hardness of 105–140 N, 40% indentation hardness of 130–150 N, rebound rate of 50–65% and fine pore structure. The qualified U-PUF displays outstanding homogeneous adsorption performance, reaching adsorption equilibrium within 30 min with an optimal adsorption capacity of 3.91 mg g⁻¹, and its adsorption kinetics are in accordance with the intraparticle diffusion and pseudo-second-order kinetic models. The gold enrichment mechanism is proposed: the secondary amine groups on U-PUF are protonated in the acidic system to electrostatically capture [AuCl₄]⁻ anions, and the adsorbed Au(iii) is further reduced in situ to form elemental gold aggregates with sizes ranging from 300 nm to 15 µm.Method validation results demonstrate that the detection data obtained by the screened U-PUF are highly consistent with the certified values of reference materials and the results of conventional methods (activated carbon enrichment and fire assay). This study solves the key pain point of the batch screening of commercial U-PUF, and provides a solid technical support for the standardized application of U-PUF in Au enrichment. In the future, the established screening criteria will be extended to precious metal analysis in complex matrices, while chemical characterization (e.g., Py-GC-MS, XRF) of additives in colored U-PUF will be pursued to further refine these physical property standards.

Author contributions

Cang Gong: conceptualization, methodology, formal analysis, data curation, writing – original draft, visualization. Yang Chen and Haichuan Lu: methodology, software, formal analysis, investigation, data curation. Shunxiang Wang: conceptualization, writing – review & editing, supervision. Jiufen Liu: conceptualization, investigation, supervision, writing – review & editing. Lihua Wang: conceptualization, investigation, supervision, writing – review & editing. All authors have approval to the final version of the manuscript.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

Data will be made available on request.