نوع مقاله : مقاله پژوهشی
نویسندگان
گروه علوم خاک، دانشکده کشاورزی، دانشگاه شهید باهنر کرمان، کرمان، ایران
چکیده
کلیدواژهها
موضوعات
عنوان مقاله [English]
نویسندگان [English]
Background and Objectives: Leonardites, organic materials rich in humic substances derived from decomposed plant residues, are key sources for humic acid (HA) extraction, which is widely used in sustainable agriculture to enhance soil structure, nutrient uptake, and microbial activity while reducing heavy metal absorption. Recent studies indicate that humic acid can enhance plant growth by up to 25% while also improving soil health. Although leonardites from regions such as Poland and Chile have been widely investigated, revealing considerable chemical variability, limited information is available on Iranian leonardites. In particular, the Pabedana deposit (Zarand, Kerman) remains poorly characterized, highlighting the need for a comprehensive evaluation to support its potential agricultural applications.This study aimed to characterize the chemical and structural properties of leonardites and extracted HA from the Goltoot coal mine in Pabedana, assessing their potential for soil improvement through extraction yields, structural ratios, elemental composition, and functional groups.
Methodology: Sixteen samples (13 leonardites, L1–L13; and 3 coal-washing tailings, C1–C3) were collected from the Goltoot mine (30.87°N, 56.37°E, at an elevation of 2000–2500 m). Samples were air-dried, and pH/EC were measured (at a 1:20 ratio). HA was extracted using the IHSS California method: a 10 g sample (<75 μm) was mixed with 0.1 M NaOH, agitated for 24 hours, filtered, acidified to pH 1–2, washed, and dried. Extraction efficiency was calculated as (HA weight/sample weight) × 100. UV–Vis spectrophotometry assessed E4/E6 (aliphatic-to-aromatic ratio), E3/E5 (molecular size), and E2/E3 (molecular weight). Six representative high-yield samples were analyzed using elemental CHNS analysis and FTIR spectroscopy (400–4000 cm⁻¹) to characterize their functional groups.
Results: Extraction yields were highest for L1 (61.64%), L2 (60.05%), L10 (60.07%), and L11 (62.35%), but lowest for L7 (15.56%) and the tailings (C1: 19.11%, C3: 20.55%). Leonardite pH ranged from 2.59 (L5) to 8.38 (C3), and EC ranged from 215 (L13) to 1031 μS cm⁻¹ (L8). HA pH varied from 1.50 (L6) to 7.10 (L9), and EC ranged from 63 (C3) to 1035 μS cm⁻¹ (L6). E4/E6 ratios indicated aliphatic structures in L3 (3.93) and L6 (3.88), and aromatic structures in C1 (0.12) and L13 (0.58). E3/E5 ranged from 0.78 (L9) to 5.07 (L11), showing molecular size variation; E2/E3 ranged from 0.004 (L3, L6) to 0.08 (L2, L7, L12), indicating molecular weight differences. Elemental analysis showed C (35.29–57.06%), H (2.30–4.34%), N (1.97–2.35%), and S (up to 3.34% in L5). FTIR spectra confirmed increased carboxylic (1700–1720 cm⁻¹) and reduced phenolic (800–900 cm⁻¹) groups, with stable aromatic structures (1500–1600 cm⁻¹).
Conclusion: High yields in samples such as L1, L2, L10, and L11 indicate a rich humic content with minimal impurities, facilitating efficient extraction. In contrast, lower yields in tailings and L7 may result from oxidation or mineral interference. Samples dominated by aliphatic structures (e.g., L3 and L6) show stronger potential for soil interactions, while those with higher aromatic content (e.g., C1 and L13) provide greater structural stability. Elemental differences, such as elevated hydrogen in L1 or sulfur in L5, highlight their roles in nutrient release and stress resistance. Variations in pH and EC further suggest targeted applications, with acidic humic acids being suited for alkaline soils and low-EC being fractions more favorable for saline soils. FTIR results further confirm oxidation-driven humic acid formation, consistent with broader patterns observed in similar materials. Pabedana leonardites, especially L1, L2, L10, and L11, are promising HA sources with high yields and diverse properties suitable for sustainable agriculture. Their chemical variability supports applications in nutrient management and soil remediation, but field trials and extraction optimization are needed to confirm their efficacy.
کلیدواژهها [English]
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