حذف فسفر از محلول آبی توسط هیدروکسید دوگانه لایه ای منیزیم-آهن

تجمع فسفر در محیط های آبی باعث به خطر افتادن حیات موجودات زنده می شود. از این رو، ارائه یک روش موثر جهت حذف فسفر از محلول های آبی ضروری به نظر می رسد. مطالعه حاضر با هدف تولید یک کانی هیدروکسید دوگانه لایه ای منیزیمآهن و بررسی رفتار آن در جذب فسفر از محلول آبی انجام شد. کانی مورد نظر به روش هم رسوبی در فاز مایع با ترکیب کلریدمنیزیم و کلریدآهن با نسبت مولی 3 به 1 در ph=10 در آزمایشگاه تهیه و ساختار بلوری آن توسط آنالیز پراش پرتو ایکس مورد مطالعه قرار گرفت. قابلیت جذب فسفر توسط هیدروکسید دوگانه لایه ای منیزیمآهن تولید شده با انجام آزمایشات تعادلی در حالت بسته بررسی و اثر عواملی نظیر غلظت اولیه فسفر، ph و زمان تماس مطالعه گردید. نتایج نشان داد که فرآیند جذب فسفر توسط هیدروکسید دوگانه لایه ای یک فرآیند وابسته به ph بوده و بیشترین جذب در ph=4 اتفاق افتاد. زمان تعادل برای واکنش جذب فسفر 60 دقیقه بود و سینتیک جذب به خوبی توسط مدل شبه رده دوم توصیف گردید. داده های هم دمای جذب فسفر به خوبی با مدل لانگمویر برازش یافتند و حداکثر ظرفیت جذب فسفر 13.96 میلی گرم بر گرم محاسبه شد. مطالعات سینتیکی و اثر ph بر روی جذب فسفر نشان داد که مکانیسم های احتمالی درگیر در فرآیند جذب فسفر به صورت برهم کنش الکترواستاتیک، تبادل لیگاندی و تشکیل کمپلکس سطحی بود. نتایج این پژوهش نشان داد که هیدروکسید دوگانه لایه ای منیزیمآهن به عنوان یک جاذب کم هزینه و دوست دار محیط زیست قابلیت موثری در حذف فسفر از محلول های آبی دارا می باشد.

Removal of Phosphorus from Aqueous Solution by Mg-Fe Layered Double Hydroxide

Introduction Phosphorus (P) is an essential nutrient for all forms of life on the earth, but in excess concentrations, it can act as a serious water pollutant through eutrophication. Thus, it is very important to remove P from aqueous solutions before their release into natural water resources. Among the various P removal techniques that have been developed, the sorption process is widely accepted to be an effective water treatment technique because of low cost, ease of operation, simplicity of design, and high sorption capacity in dilute solutions. Layered double hydroxides (LDHs) are a type of twodimensional nanostructure anionic clays with high capacities to sorption of anions. These nonsilicate clays consist of positively charged brucitelike octahedral sheets which neutralize by a negatively charged interlayer containing relatively weak bonded anions and water molecules. The positive charges generated by the isomorphous substitution of trivalent cations for divalent cations are balanced by interlayer anions that can be exchanged by other anions making them good anionexchangers with high selectivity. LDHs have been widely used as environmental sorbents because of their high charge density, large interlayer areas, good thermal stability, and high anion exchange capacities of the interlayer anions. The aim of the present study was to synthesize a MgFe LDH as a sorbent for P removal from aqueous solution. Materials and Methods The MgFe LDH was synthesized using the coprecipitation method. In brief, a mixture solution containing 0.03 mol MgCl2. 6H2O, and 0.01 mol FeCl3. 6H2O was added dropwise into a flask containing 100 ml of 1 M NaOH solution under vigorous stirring at pH=10. The obtained slurry was filtered and washed repeatedly with DW until the filtrate pH reached neutral. MgFe LDH particles were then obtained by drying the filtrate at 70 °C in an oven overnight. The crystallinity of the sample was studied using Xray diffraction (XRD) analysis. In order to investigate the performance of the synthesized LDH as a P sorbent, batch experiments were carried out in polyethylene centrifuge tubes. The suspensions were shaken for 24 hours at 250 rpm, and the supernatant was then separated by centrifugation at 4000 rpm for 10 minutes and were filtered by Whatman ashless grade 42 filtration papers. Equilibrium P concentration was determined according to the ascorbic acid method using UVvis spectrophotometer at the wavelength of 880 nm. The effects of pH, initial P concentration, and contact time on P sorption were investigated in the ranges of 210, 0300 mg/L and 01440 min, respectively. Results and Discussion The XRD pattern of the LDH sample showed typical structure of hydrotalcitelike compounds with sharp and reflection peaks corresponding to the (003), (006), (012), (015), and (110) crystal planes which are characteristic planes of hydrotalcitelike compounds. The efficiency of LDH to remove P decreased with the increasing of initial P concentration and the maximum removal efficiency of LDH occurred in the range of 520 mg/L of initial P concentration. With increasing of initial P concentration from 20 to 300 mg/L, the P removal efficiency of LDH decreased from 98.7 to 24.6 %. The P removal efficiency was increased with time and reached equilibrium at 60 min. The P removal rate of LDH in this time was about 66 % and no significant decrease in residual P concentration was observed after 60 min. The sorption of P on LDH was highly pH dependent, and the maximum P removal was found at pH of 4. The sorption kinetic and isotherm data were well described by pseudosecondorder and Langmuir equations, respectively. According to the Langmuir equation, the maximum P sorption capacity (Qmax) of LDH was obtained as 13.96 mg/g. Conclusion It was found from the results of this study that the mechanisms involved in the P sorption onto LDH included electrostatic attraction, ligand exchange, and surface complex formation. In addition, the results suggested that the synthesized MgFe LDH can be potentially used as an effective sorbent for the removal of P from aqueous solutions. Further research is needed on the regeneration of the LDH after P sorption and the evaluation of desorption behavior of P from LDH under different conditions.