Preparation of Spent Mushroom Substrate Biochar and Its Application for Gold Determination in Rock and Mineral Samples by Flame Atomic Absorption Spectrometry

BRIEF REPORTSignificance: Accurate gold analysis is critical for geological exploration and ore deposit genesis interpretation. However, the traditional activated carbon adsorption-flame atomic absorption spectrometry (FAAS) method fails to meet analytical requirements due to the high ash content and low adsorption rate (only 90%–92%) of conventional activated carbon. In this study, agricultural waste (spent mushroom substrate, SMS) was used to prepare biochar. After optimization via response surface methodology (RSM), the gold adsorption rate exceeded 98%. The relative error of gold ore certified reference materials (CRMs) ranged from 0.05% to 1.59%, and the recovery rate of real samples was 96.8%–101.9%, both complying with geological analysis standards. This study provides a new, efficient, and environmentally friendly solution for gold enrichment.Introduction: As a strategic mineral, the analytical accuracy of gold directly affects the estimation of resource reserves and the interpretation of ore deposit genesis in geological exploration[1-2]. With the expanding application of gold in new energy and electronics fields, the demand for high-precision analysis of gold in geological samples has become increasingly urgent, and the efficiency of the enrichment step is a core bottleneck affecting the reliability of analytical results.　　The current mainstream enrichment methods for gold in geological samples are foam plastic adsorption and activated carbon adsorption: the former exhibits large fluctuations in adsorption efficiency and is only suitable for samples with low gold content[5-6]; the latter (coal-based, coconut shell-based, etc.) has high ash content and low adsorption rate (≤92%), with limited supply of qualified products[7-8]. Although SMS-derived biochar exhibits excellent performance in adsorbing environmental pollutants due to its high specific surface area and porous structure, such as Lu et al.[13] confirmed its good adsorption capacity for high-molecular-weight polycyclic aromatic hydrocarbons (PAHs); however, existing studies have primarily focused on the treatment of water pollutants and have not involved gold analysis and enrichment. Moreover, most studies only regulate the preparation process through single factors, lacking systematic optimization of key parameters affecting adsorption efficiency[9-10], which cannot meet the demand for gold determination in geological samples.　　To address the problems of low adsorption rate of traditional materials and the unused application of SMS-derived biochar in gold enrichment, we used SMS from Pleurotus eryngii and Pleurotus ostreatus as raw materials to prepare biochar via anaerobic pyrolysis-ultrasonic hydrochloric acid modification. Four parameters (carbonization temperature, ultrasonic time, modifier concentration, and adsorption time) were optimized using RSM with Box-Behnken design. A gold analysis method was established by combining with FAAS. The applicability of the method was verified through 4 types of gold ore CRMs (including GBW07807a, covering low/medium/high content) with 6 parallel determinations, and spiked recovery tests of actual gold ore samples from Gannan, Gansu Province.Methods: SMS was the residue from the production of Pleurotus eryngii and Pleurotus ostreatus. After drying, it was crushed to 40–60 mesh and then subjected to anaerobic pyrolysis (505℃, 0.5 h), followed by ultrasonic modification with 9% hydrochloric acid (20℃, 100 kHz, 54 min), suction filtration and washing to neutrality, and a final secondary pyrolysis. Finally, it was ground to 200 mesh to obtain SMS-derived biochar. Four types of gold ore CRMs (including GBW07807a and GBW07297a) and four actual gold ore samples (S1–S4) from Gannan, Gansu Province, China, were used.　　Reagents included a 1000 mg/L gold standard solution (GSB 04-1715-2004), superior grade pure hydrochloric acid (1.19 g/mL), nitric acid (1.42 g/mL), and ultrapure water (resistivity ≥18.25 MΩ·cm). Instruments used were a Z-2000 FAAS (Au wavelength: 242.8 nm, lamp current: 7.0 mA, fuel gas flow rate: 1.8 L/min), box-type resistance furnace (SX2-12-12), constant-temperature ultrasonic instrument, and planetary ball mill.　　The ranges for carbonization temperature (350–650℃), ultrasonic time (1–120 min), modifier concentration (1%–20%), and adsorption time (10–300 min) were determined through single-factor experiments. Using the gold adsorption rate as the response, a four-factor, three-level Box-Behnken design (Table 1) was employed for RSM optimization to construct a quadratic polynomial regression model. Referring to GB/T 20899.1-2019, 6 parallel determinations of CRMs were conducted to calculate accuracy and precision. Spiked recovery tests were carried out on the real samples. All results were evaluated for compliance with DZ/T 0130.3-2006.Data and Results: The results are presented in the following three parts.　　(1) RSM optimization results. Analysis of variance (ANOVA) of the RSM model indicated that the model was extremely significant (F = 339.27, P < 0.0001) with a non-significant lack of fit (P = 0.6357). The fitting degree between actual values and predicted values was excellent (Fig. 1, r = 0.99853). The optimal preparation parameters were as follows: carbonization temperature, 505℃; ultrasonic time, 54 min; modifier concentration, 9%; and adsorption time, 170 min. Under these parameters, the gold adsorption rate reached over 98% (Table 2).　　 (2) Method validation results. In the determination of gold ore CRMs, the relative error ranged from 0.05% (GBW07810) to 1.59% (GBW07807a), and the relative standard deviation (RSD, n = 6) ranged from 0.30% (GBW07299a) to 3.41% (GBW07807a) (Table 4). The spike recovery rates for the real samples from Gannan, Gansu ranged from 96.8% (S1) to 101.9% (S2) (Table 5), all meeting the relevant industry quality requirements.　　(3) Factor interaction analysis. Response surface and contour plots (Fig. 2) revealed that all four factors significantly affected the gold adsorption rate (P < 0.05), with adsorption time exhibiting the greatest influence (regression coefficient: 14.22). Significant interactions between factors were observed (evident from elliptical contour lines). For instance, a higher carbonization temperature enhanced the positive effect of adsorption time on the adsorption rate, and ultrasonic treatment improved the modification efficiency of hydrochloric acid.