بررسی همدمای جذب روی و مس بر سطح اسید هومیک استخراج شده از خاک

عناصر مس و روی یکی از عناصر کم مصرف لازم برای رشد گیاهان است که قابلیت استفاده آن ها توسط بسیاری از عوامل تحت تاثیر قرار می‌گیرند. فرآیندهای جذب نقش تعیین کننده بر حلالیت عناصر مذکور در محلول خاک و درنتیجه دردسترس بودن آن ها برای گیاهان ایفا می کند. اسیدهومیک مهم ترین بخش آلی خاک بوده که دارای توانایی جذب فلزات می باشد. هدف از این تحقیق استخراج اسیدهومیک خاک، مطالعه جذب سطحی عناصر بر سطح اسیدهومیک و تعیین ضرایب جذب سطحی آن ها با استفاده از همدماهای جذب می باشد. در این پژوهش ده نمونه خاک سطحی از مناطق جنگلی استان گیلان جمع آوری شد. اسیدهومیک موجود در خاک ها با استفاده از محلول های 1/0 مولار naoh و 6 مولار hcl استخراج و با 1/0 مولار hcl + 3/0 مولار hf خالص سازی شد. همدمای جذب عناصر (فلزات) مس و روی با به تعادل رساندن اسید هومیک با محلول هایی با غلظت 10، 20، 40، 60 و 80 میلی گرم بر لیتر مس و روی بترتیب از منابع  cuso4و  zncl2در محلول زمینه ca(no3)2 01/0 مولار بدست آمد. نمونه ها به مدت 12 ساعت در 5ph= و دمایc°25 در شیکرانکوباتور تکان داده شدند، سپس نمونه ها سانتریفیوژ و مایع رویی از کاغذ صافی عبورداده و توسط دستگاه جذب اتمی قرائت شد. داده های آزمایشی بر سه مدل همدمای لانگمویر، فروندلیچ و تمکین برازش داده شدند. دقت برازش داده ها براساس معیارهای ارزیابی r2 وrmse نشان داد که مدل فروندلیچ جذب مس و روی را به خوبی توصیف می کند. با انجام مقایسه میانگین بین مقادیر حداکثر جذب محاسبه شده از مدل لانگمویر نشان داد که جذب مس بیشتر از روی است.

Study of Adsorption Isotherm of Copper and Zinc on Humic Acid Extracted from Soil

Introduction: Copper and zinc are two of the most important microelements affecting plant growth which can be influenced by many factors. The adsorption processes play a determinative role in solubility of copper and zinc elements in the soil solution and, therefore, their availability to plants. Organic matter is one of the most important factor that have an significant role on the absorption and desorption of elements in the soil. These materials are divided into humic and nonhumic groups. Humic substances are divided into three groups of fulvic acid, humic acid and humic, based on their resistance and solubility in acid and base. Humic acid with a medium molecular weight and color is soluble into base and insoluble into acid, and has a medium resistance against the microbial attack. It forms the most important organic part of the soil and is capable to adsorb metals. The purposes of this study were to extract soil humic acid, study the adsorption of metals on the surface of humic acid and to determine the metals adsorption coefficients using adsorption isotherm models. Materials and Methods: Sampling was done from forest areas of northern Iran. Some physical and chemical properties of the studied soil were determined. Then, the humic acid of the soils was extracted by 0.1 M NaOH and 6 M HCl, and purified by 0.1M HCl+0.3M HF. Functional group, E4/E6 ratio (Optical density or absorption of dilute solutions at wavelengths of 465 and 665 nm), and humification index of the extracted humic acid were measured. Some other properties of the extracted humic acid have also been analyzed. To study the adsorption isotherms of Zn and Cu in the presence of humic acid, solutions with concentrations of 10, 20, 40, 60 and 80 mg/L of  ZnCl2 (zinc adsorption testing) and CuSO4 (copper adsorption testing) were prepared, respectively in a 0.01M Ca(NO3)2 background solution, and added to 250 g of  humic acid. The samples were shaken for 12 hours (based on the time of equilibrium) at pH=5 and 25 °C in incubator shaker, then the samples were centrifuged and the supernatant was passed through filter paper and measured using atomic adsorption spectrophotometer device. The difference between initial concentration and final concentration identified the amount of adsorbed element. Results and Discussion: The results of the acidic functional groups measurement in the humic acid samples revealed that the most of total acidity (60%) was due to the presence of phenolic groups while the carboxylic groups were responsible for the remaining (40%). Phenolic groups were abundant in the primary stages of the decomposition of humic materials. Since the soil used for extracting humic acid was covered with broad leaf trees and the continuous entry of organic matter into it (the fall of leaves) lasted for many years and due to the low temperature of the soil in part of the year, it can be said that a significant part of the soil organic matter is in the primary or middle stages of humification and the phenolic OH groups/carboxylic groups ratio in the humic acid extracted from them was high. The equilibrium time for adsorption of both  metals occured at 12 h to achieve maximum adsorption level in the presence of humic acid. The obtained experimental data were fitted to three models of Langmuir, Freundlich, and Tampkin. The accuracy of mentioned models to fit data were estimated based on the detection coefficient (R2) and the roots of mean square error (RMSE). The results showed that the Freundlich model with higher detection coefficient and lower roots of mean squared error describes the adsorption of copper and zinc elements, well. To better compare the adsorption of the elements by humic acid, Langmuir’s b parameter (Expresses maximum adsorption) can be used. The maximum adsorption of copper (23.04 mg/g) by humic acid was higher than zinc adsorption (13.8 mg/g). This trend is consistent with the Irving–Williams series of divalent elements: Mn < Fe < Zn < Co < Ni < Cu. It is generally believed that humic acid is a good complexing agent for many metal ions and its binding to metal ions can improve the adsorption. Significance differences were tested by a parametric 𝑡test or 𝐹 statistics in ANOVA (analysis of variance). There was a significant correlation between the maximum adsorption of metals (b) and the properties of humic acid at a probability level of 5%.