تأثير مواد زائد حاصل از پالايش نفت خام بر برخي ويژگي‌هاي فيزيكي خاك

Impact of Waste Materials Resulting from the Refining of Crude Oil on Some Soil Physico-Chemical Properties

آلودگی خاك و آب‌های زیرزمینی به مواد آلی و سموم یك مشكل شایع زیست محیطی است .در این میان مشتقات نفتی یكی از مهم‌ترین آلوده سازهای محیط زیست محسوب می شوند. در اثر فرآیندهای فیزیكی و شیمیایی كه بین آلاینده و خاك رخ می دهد، رفتار خاك تغییر می كند. جهت بررسی برخی از این تغییرات چهار نیمرخ آلوده و دو نیمرخ غیرآلوده مجاور با خاك های آلوده با مواد زائد حاصل از پالایش نفت خام مورد مقایسه قرار گرفتند. ویژگی‌های فیزیكی، شیمیایی، كانی شناسی و میكرومورفولوژیكی نمونه های دست خورده و دست نخورده خاك طبق روش های استاندارد بررسی شدند. اثرات قابل توجه مواد آلاینده در خاك شامل كاهش pH و افزایش كربن آلی و قابلیت هدایت الكتریكی می باشد. حد روانی و حد خمیرایی در خاك‌های آلوده به تركیبات زائد نفتی نسبت به خاك‌های غیرآلوده افزایش یافته است. با افزایش مواد آلاینده در خاك‌های آلوده به نفت، شاخص خمیرایی كاهش یافت. نتایج میكرومورفولوژی و كانی شناسی رس خاك نیز نشان داد افزایش آلاینده در خاك موجب افزایش فضای بین لایه ای كانی‌های خاك می گردد. بعلت ثابت دی الكتریك پایین نفت، نه تنها خاك دیسپرس نشده بلكه باعث فلكوله شدن و تشكیل خاكدانه-های گرانولار شده است. خاك های مورد مطالعه از نظر شاخص فعالیت كانی‌های رسی، غیرفعال بود. نتایج نشان داد این خاك ها حتی جهت بهره-برداری جنبی دارای محدودیت هستند بطوریكه در خاك‌های آلوده به نفت بدلیل محدوده كمی كه بین حد خمیرایی و روانی وجود دارد امكان لغزنده شدن آن‌ها سریعتر از خاك‌های غیرآلوده است و باید از ساخت و ساز اجتناب شود.

Introduction: Soil and ground water pollution with organic matter and toxic materials is an ordinary environmental problem. In this case, oil compounds are among the most important environmental pollutants. Tehran oil refinery is one of the largest and oldest refineries in Iran located south of Tehran city. Since the beginning of its activity in 1968, its waste materials (solid, semisolid and liquid) have been disposed in large lagoons next to the refinery site. During this long period, considerable changes in soil properties have occurred, which are of great research interests for soil and environmental scientists. Materials and Methods: The studied area (about 60 ha) was located in the south of Tehran (latitude: 35°30.299 ' to 35°30.814 ' N and longitude: 51°25.682 ' to 51°26.296 ' E). Six pedons, including four Technosols developed on the oil refinery waste materials (pedons no. 1, 3, 4 and 6) and two reference pedons (pedons no. 2 and 5) were fully described and sampled. Particle size distribution (PSD) of gypsiferous samples was determined by the specific method for gypsiferous soils (Hesse, 1976). PSD of non-gypsiferous samples were determined according to the standard hydrometer method (Gee and Bauder, 1986), but the oil-polluted samples were analyzed according to the standard ultrasound method (Sawhny, 1996). Organic carbon content was determined by Walkley and Black (1934). pH and EC were measured in soil saturation extracts using EC and pH meter (Jenway). Gypsum and CaCO3 contents were determined using acetone (Sparks, 1996), and calcimetery methods, respectively. Mineralogical analysis was done by Decarreau (1990). Micromorphological descriptions were carried out using the terminology of Stoops (2003). Diagnostic horizons were identified and finally the studied pedons were classified according to the Keys to Soil Taxonomy (Soil Survey Staff, 2014) and the World Reference Base (FAO, 2014). Results and Discussion: Horizons of both polluted and unpolluted soils were mostly gypseous and/or calcareous especially in the middle parts. Considering the surface and subsurface diagnostic horizons and the aridic-thermic soil moisture and temperature regimes, the studied soils were classified as Gypsids, Calcids or Cambids (Soil Survey Staff, 2014). However, due to the added oil waste compounds and presence of impermeable geomembrane in some of the polluted pedons, they were classified as Technosols in the WRB system (FAO, 2014). Noticeable effects of Pollutants in the soil were decreasing pH and increasing OM and EC. The surface horizons of the unpolluted soils contained less than 2 percent organic matter which regularly decreased by depth. However, In some horizons of the polluted soils, soil organic matter exceeded 12 percent. pH decreased by increasing organic matter (oil waste compounds) possibly due to H+ dissociation from the oil compounds (Laurent et al., 2012). Electrical conductivity throughout the polluted soil horizons showed more limited variability than the unpolluted ones, probably due to their higher capability in water and liquid dynamics. Liquid limit and plasticity limit in polluted soils are higher than unpolluted soils . Plasticity index in polluted soils decreased with increasing the amount of pollutants. The results of mineralogical studies corroborated that dominant clay mineral in this soils is Smectite. Smectites have high swelling and shrinking capacity.So, the pollutants can intercalate between soil mineral layers and then increase d-spacing of clay minerals. The micromorphology of the polluted soils showed that low dielectric constant of petroleum caused flocculation and formation granular aggregates in soil. Studied soils are inactive in terms of activity of clay. Conclusion: As observed in the studied soils, their physico-chemical properties such as pH and electrical conductivity (EC) of saturated paste extract, organic matter content, mineralogical and micro-morphological properties were severely affected by the oil wastes. These results show these soils have been limited and in use of these soils, considering these properties is required. In polluted soils, due to small differences between plasticity limit and liquid limit, it is possible to their sliding faster than unpolluted soils and then construction should be avoided.