كليدواژه :
كادميوم , بنتونيت , سيمان , نانو سيليس , جامدسازي , pH
چكيده فارسي :
در سال هاي اخير استفاده از نانو مواد در پروژه‌هاي مختلف علوم و مهندسي مورد توجه قرار گرفته است. در اين راستا، مطالعه تأثير نانو مواد در تركيب با ديگر مصالح از زمينه هاي نوين در علوم مهندسي ژئوتكنيك و ژئوتكنيك زيست‌محيطي است. اين تحقيق به منظور تعيين مكانيزم نگهداشت آلاينده فلز سنگين كادميوم در فرآيند تثبيت و جامدسازي پايه سيماني بنتونيت آلوده درحضور نانو سيليس انجام شده است. مكانيزم نگهداري آلاينده با بررسي نتايج آزمايشهاي تعيين رفتار كادميوم و نانو سيليس با تغيير pH محيط، جذب، آبشويي آلودگي (TCLP) و منحني هاي پراش پرتو ايكس (XRD) تجزيه و تحليل شده است. نتايج نشان مي دهد كه قابليت جذب و نگهداري فلز سنگين كادميوم توسط بنتونيت، نسبت به ساير فلزات سنگين، از جمله سرب، مس و روي، كمتر است. با توجه به روند افزايش مقدار كادميوم آبشوييشده از آزمايش TCLP در نمونه هاي حاوي درصدهاي مختلف سيمان، مي توان نتيجه گيري نمود كه معيار 28 روز پيشنهاد شده توسط استاندارد EPA، براي تثبيت و جامدسازي كادميوم، مناسب و مطمئن نبوده و لازم است اين آزمايش را در فاصله زماني بيشتري انجام داد و سپس نتايج آن مورد استفاده قرار گيرد. همچنين ارزيابي نتايج آزمايشهاي اشعه‌ايكس بيانگر پيشرفت موثرتر واكنش‌هاي پوزولاني در حضور نانو سيليس است. ميزان كادميوم آبشويي شده از آزمايش TCLP، با توجه به حضور درصد زياد سيمان و نانو سيليس كه به تبع داراي ميزان قابل توجهي C-S-H هستند، نشان مي دهد كه نقش اصلي در نگهداشت آلاينده فلز سنگين كادميوم، فرآيند رسوب هيدروكسيد و مقدار pH قليايي محيط به طور توام با اثر جامدسازي C-S-H است
چكيده لاتين :
In recent years, the use of nano-materials in different engineering and science projects has increased. The study of
the impact of nano-materials in combination with other civil engineering constituents in different geotechnical and
geo-environmental engineering projects is very common. This study is aimed to investigate the mechanism of the
cadmium retention in the process of cement based solidification/stabilization of cadmium contaminated bentonite in
the presence of nano-silica. The mechanism of contaminant retention is investigated with the evaluation of the
cadmium and nano-silica behaviour with change in pH of the environment, adsorption, TCLP results, and evaluation
of XRD experimental achievements. The bentonite sample for this research is taken from Iran-Barit Company. To
establish the availability of silica ions for interaction with cement and bentonite at different pH, a series of solubility
experiments of nano-silica at different pH levels were performed. The results of solubility experiments show that as
the pH increases to the alkaline range, the solubility of nano-silica noticeably increases. This fact proves that at the
high range of pH due to the use of cement, the required pH conditions for solubility of nano-silica will be provided.
Therefore, there will be more possibility for the formation of CSH component. Cadmium nitrate was used to
contaminate the bentonite sample for the experimental part. For this purpose, bentonite samples were mixed with 10,
30, and 50 cmol/kg-soil of cadmium nitrate in the electrolyte soil ratio of 20:1. Then, these samples were shaken for
two hours in every 24 hours. This process was repeated for 96 hours. After this equilibrium step, the soil suspension
was centrifuged. After drying these laboratory contaminated samples, they were solidified/stabilized with different
percentages of cement and nano-silica. The results of this paper indicate that the contaminant adsorption and
retention of cadmium by bentonite is less than that of adsorption for zinc and lead. The achieved results of TCLP
experiments for solidified/stabilized samples with different percentages of cement indicate that the EPA criteria for
TCLP experiment which emphasizes for test performance after 28 days, is not suitable for solidification and
stabilization of cadmium. In fact, a longer period is necessary to achieve equilibrium and stable results. Furthermore,
the results show that due to the low adsorption of cadmium by bentonite and due to the noticeable reduction of pH in
the presence of cadmium ions, the required percentages of cement for solidification/stabilization of cadmium
contaminated bentonite is much more than the required quantity of cement for other heavy metal contaminated
bentonite samples. In addition, the results of XRD experiments show that the pozzolanic interaction process is more
efficient in the presence of nano-silica. Furthermore, based on the results of TCLP experiments, the formation of
CSH in the presence of nano-silica contributes to the contaminant retention by solidification/stabilization of cement
based cadmium contaminated bentonite. Finally, according to the results of this study, in solidified/stabilized
samples by mixtures of cement and nano-silica, it is shown that due to the contribution of silica ions in pozzolanic
interactions, the solidification is the governing phenomenon for the prevention of heavy metal leachate from
solidified/stabilized samples.
