تاثير برخي مواد اصلاحي آلي و معدني بركاهش زيست فراهمي و توزيع شكل هاي شيميايي سرب و كادميوم در يك خاك آهكي آلوده به عناصر سنگين

Effects of some organic and inorganic amendments on the bioavailability and distribution of different Fractions of lead and cadmium in a calcareous contaminated soil

آلودگي منابع آب و خاك به فلزات سنگين، از چالش­هاي مهم عصر كنوني است؛ به­ همين دليل، آلودگي­زدايي چنين خاك­هايي، پيش­نياز هرگونه بهره ­برداري بهينه از اين منابع است. يكي از روش­هاي مقرون به­صرفه براي پيش­گيري از انتشار فلزات سنگين در منابع آب و خاك، تثبيت و جامدسازي آن­هاست. طي اين فرايند، فلزات سنگين در خاك­هاي آلوده با اصلاح­كننده­ هايي همچون مواد آلي و معدني واكنش داده و با تشكيل موادي كم­محلول يا نامحلول در محيط، به صورتي پايدار باقي­ مي­مانند. در اين پژوهش، به‌منظور بررسي اثر مواد اصلاحي آلي (بيوچار تهيه شده در دماي 640 و زمان 30 دقيقه از كاه و كلش برنج و بيوچار تهيه شده در دماي 420 و زمان 2 ساعت از كاه و كلش برنج) و معدني (پوميس، ليكا، زئوليت و بنتونيت) در سه سطح صفر، 1 و 5 درصد وزني بر عدم تحرك و تثبيت دو فلز كادميوم و سرب در خاك­هاي آلوده، آزمايشي در قالب طرح كاملا تصادفي با سه تكرار انجام گرفت. نتايج حاصل از اين پژوهش نشان داد كه افزودن مواد اصلاحي به خاك، موجب كاهش غلظت كادميوم و سرب عصاره ­گيري شده با DTPA و EDTA شد. بيشترين كاهش غلظت سرب عصاره­ گيري شده با DTPA و EDTA (بخش محلول يا زيست فراهمي) در تيمار سطح 5٪ بيوچار 640 مشاهده شدكه در مقايسه با شاهد، به ­ترتيب كاهشي معادل 68 و 41/2 درصد داشتند؛ همچنين بيشترين كاهش غلظت كادميوم عصاره­گيري شده با DTPA و EDTA به ­ترتيب مربوط به تيمارهاي سطح 5٪ پوميس و زئوليت بود كه به ترتيب 39 و 28/8 درصد نسبت به شاهد كمتر بودند. بيشترين كاهش غلظت سرب و كادميوم در بخش تبادلي در سطح 5٪ بيوچار 640 مشاهده شد كه در مقايسه با شاهد، به ترتيب كاهشي معادل 54/96 و 41 درصد داشتند. كاربرد اصلاح­ كننده ­ها، اثري معني­دار بر مقدار كل سرب و كادميوم خاك نداشتند. بنابراين، تكنيك حاضر، مي­تواند به طور مؤثري، زيست فراهمي سرب و كادميوم را در خاك كاهش دهد كه اين مساله مي­تواند در اصلاح خاك استفاده شود.

Introduction At present, contamination of water and soil resources is an important environmental challenge. Therefore, decontamination of such is a prerequirement for using these resources. Cadmium (Cd) and lead (Pb) often coexist in contaminated soils and there is currently no effective means for their concurrent removal. Concerns about their mobility and bioavailability have increased because of food safety, potential health risks and its detrimental effects on the ecosystems. The stabilization/solidification is a cost effective remediation method that prevents spreading of heavy metals in soil and water resources. In this process, contaminated soil reacts with amendments such as organic and liming materials to form low soluble or non-soluble stable materials. The objective of this study was to evaluate the effect of several low cost amendments on Cd and Pb stabilization by a sequential extraction method. Materials and Methods In this research, in order to investigate the effect of organic amendments (biochar 640°C, and biochar 420°C) and inorganic amendments (Pumice, Leca, Zeolite, and Bentonite) on Pb and Cd stabilization in a contaminated soil, an incubation experiment was carried out. One kilogram of each amended soil and the control soil were packed into respective pots. Soils were amended in the laboratory using biochar 640 (BI1), biochar 420 (BI2) bentonite (BE), pumice (P), leca (LE), and zeolite (Z). A control treatment (C) without adding amendment was also prepared. The amendment materials were applied at 1 and 5 percent wt. Each treatment was performed in three replicates and the samples were incubated in the dark at 14°C for 6 months. At the end of the incubation time, the potential bioavailability of Cd in non-amended and amended soils was evaluated by extraction with DTPA and ethylenediamine tetraacetic acid (EDTA). Total Cd (CdT) and Pb (PbT) was extracted by aqua regia (HNO3 + HCl) extraction. The chemical fractions of Cd and Pb were determined by a sequential extraction method which is a five-step chemical fractionation based on the work of Tessier et al. (1979). All statistical analyses were performed using SAS software. Means of different treatments were compared using LSD (P ≤0.05) test. Results and Discussion The results indicated that the additions of amendments to soils reduced the concentration of DTPA and EDTA-extracted Pb and Cd. The smallest concentration of Pb-extracted DTPA and EDTA was observed in organic amendments treated soil (biochar 640°C, and biochar 420°C) and treated with 5% biochar 640°C, respectively. The high sorbent capacity of the BI used in this study could be due to its high pH, high content of organic carbon and cation exchange capacity (CEC). The highest decreasing rate of DTPA and EDTA-extractable of Cd was observed in treated with 5% pumice and zeolite, respectively. Application of the amendments (except for 1% LE) decreased exchangeable fraction (F1) of Pb compared to the non-amended soil. Also, the amendments (except for 1% P, Z and BE) decreased exchangeable fraction (F1) of Cd compared to the non-amended soil. Although the biochar 640 (5%) showed the highest decreasing rate of exchangeable fraction (F1) of Pb and Cd, they increased the oxide (F3) and organic (F4) fractions, which might be due to its rich O-containing functional groups and high alkalinity leading to an increase in the binding of Cd and Pb to organic compounds and mineral oxides. Conclusion Results indicated that application of amendments was successful in lowering the potential bioavailability of Pb and Cd soils. The 5% biochar 640 treatment had the greatest decrease in extractable Pb. Also, the 5% zeolite and pumice treatment had the greatest decrease in extractable Cd. Application of BI resulted in a significant decrease in both Pb and Cd exchangeable fraction (F1). This reduction in the exchangeable fraction (F1) of Cd and Pb in the soil was due to an increase in the fraction of heavy metals bound to the soil organic matter (F4) oxides (F3) after BI addition. Enhanced precipitation or co-precipitation and complexation of metals with amendments led to the reduction of the solubility of the metals. The P, LE, BE, and Z altered the exchangeable fraction (F1) of Cd and Pb to the oxide fraction (F3) and the carbonate fraction (F3), respectively. Application of BI amendment causes the highest decreasing rate of solubility Cd and Pb, suggesting this as the suitable amendment for the remediation of Cd and Pb in contaminated soils.