Application of solid waste as an adsorbent for capture of 137 Cs , 85 Sr and 131 I from environmental water


 Cesium, strontium, and iodine radionuclides are produced from uranium fission and easily soluble in water through accidents or leakages, as those occurred at Chernobyl, Three Mile Island, and Fukushima. Thus, developing an economical and effective process for removing these radioisotopes from real water is a progressively important issue. Hence, adsorption of 137Cs, 85Sr, and 131I radionuclides was achieved using solid waste adsorbent. The solid waste adsorbent is granular activated carbon (GAC) gained from the wasted household water filters (from the second stage). After the adsorption process, the gained data illustrated that the percentages of uptake for 137Cs, 85Sr, and 131I were 87.6, 85.6, and 82.7 %, respectively. Application of GAC for the decontamination of real water as groundwater, river water, tap water, and seawater was achieved. The findings revealed that GAC has the ability to be employed effectively as a hopeful material for the decontamination of radioactive 137Cs, 85Sr, and 131I from the environmental water.


Introduction
Environmental and health issues produced by radioactive waste attracted attention worldwide since the wide application of nuclear science started from the early stages of the twentieth century [1]. The radioactive waste is produced from the research reactors, radiochemical labs., industrial activities, and nuclear medicine.
At the period of Chernobyl-nuclear accident, the clouds released from the reactor polluted extensive zones as they were transported by the winds, not only in the surrounding area but also in areas towards the northern hemisphere. Water-soluble radionuclides as cesium, iodine, and strontium, contaminated food products, and aquatic ecosystems. Even with the relatively short half-life of radioiodine ( 131 I), the occurrence of thyroid cancer in children existing in contaminated areas rose significantly. Consequent phases of environmental contamination were primarily due to 90 Sr and 137 Cs concentrated in human bone and muscles, respectively, due to their similarity to calcium and potassium, respectively, which led to their classification as cancerogenic.
After Fukushima accident in Japan, radioactive waste treatment has become a rising interest. At Fukushima Daiichi-Nuclear Power Plant, 15PBq of 137 Cs, 18PBq of 134 Cs, 0.14PBq of 90 Sr, 2.0PBq of 85 Sr, and 160PBq of 131 I were released to the surroundings to date, and radioisotopes from the reactors were noticed in Greece and Russia [2]. The scavenging of radiocesium is very complicated owing to the lack of knowledge of the behavior of ultra-trace ions that compete with the high amount of monovalent cations.
Different kinds of sorbents are utilized for treating radioactive waste. Low-cost adsorbents as alum-industrial waste [3], eggshell material [4], brick-kiln waste [5], marble dust [6], hydroxyapatite [7], and perlite [8] offer a probable alternative to present materials for scavenging of ions. In this investigation, the adsorptive properties of one such low-cost adsorbent (solid waste from consumed household water purifiers) is evaluated for 137 Cs, 85 Sr, and 131 I removal.
In this work, an economical method was employed to 137 Cs, 85 Sr, and 131 I removal from environmental water (groundwater, river water, and tab water) using solid waste gained from the wasted household-water filters.

Chemicals and reagents
Cesium chloride, strontiumchloride and potassium iodide salts were purchased from Merck (Merck D-6100 Darmstadt, Germany). The radioactive tracer 131 I and 85 Sr were gained from the second Egyptian Nuclear Reactor, and 137 Cs was purchased from Amersham. The solutions pH was adjusted by NaOH and/or HCl which were attained from El-NasrCo. All chemicals employed in this investigation have analytical grade purity. Aqueous solutions of 100mg/L cesium, strontium, and iodide ions were gained by dissolving their salts in bidistilledwater then labeled with 137 Cs, 85 Sr, and 131 I.

Adsorbent Preparation
The solid waste adsorbent is granular activated carbon, GAC, gained from the wasted household-water filter (from the second stage). It was renewed by 1.0mol/L nitric acid followed by hydrogen peroxide (2 g solid:1 mL HNO3 and/or 1 mL H2O2) to eliminate any dirt or metal ions adsorbed on its surface. GAC was driedfor 3h at90°C.

Batch distribution studies
Distribution coefficients (Kd) of 137 Cs, 85 where Kd is the distributioncoefficient, mL/g, Ao and Ae are the counting of liquid phase (cpm) before adsorption and after equilibration, respectively. V (mL) is the liquid volume, and m (g) is the GAC mass.

Kinetic studies
To assess the sorption kinetics, the batch was conducted by shaking 0.1g of GAC with 5.0mL of the radioactive solution of 137 Cs, 85 Sr, and/or 131 I at different interval times (1-180 min), and optimum pH value. After separation of both phases, the activity was detected in the supernatant as previously mentioned. The quantity sorbed, qt (mg/g), was estimated as follow; where Co (mg/L) is the initial ion concentration, it was analyzed by Atomic-Absorption Spectrophotometer(Buck Scientific) model 210-VGP, USA. Ao, At, m and V were identified above.

Equilibrium-isotherm-studies
Concentrations range from 50 to 500 mg/L of ions were employed to investigate the isotherm for the ions onto GAC. A 0.1g of GAC was mixed with 5.0mL of cesium, strontium and/or iodide ions, labeled by their radionuclides. The pH was adjusted to the optimum value (pH 10 for cesium and strontium ions and pH 5.0 for iodide ions), then the mixture was agitated. The activity of the filtrate was determined. The quantity sorbed of ions at equilibrium, qe (mg/g), was estimated using the previous equation after replacing At by Ae.

Effect-of-pH
The pH is the more significant variable governing the ions sorption onto sorbents. This is relatively sorbed since hydroxyl or hydrogen ions themselves powerfully compete with ions. The pH effect on sorption of anions differs from sorption of cations, therefore, the pH influence on the capture of 137 Cs + and 85 Sr 2+ cations was employed at high pH (ranged from 5.0 to 10.0) to minimize the sorption competition between the cations and hydrogen ions.
While for 131 I, low pH (ranged from 1.0 to 5) was selected to diminish the sorption competition with OHions, the outcomes are illustrated by Fig. 1. For Cs + and Sr 2+ cations, the data confirmed that the percent uptake and the distribution coefficient of both radionuclides rise by raising the pH value. The maximum percent uptake for both (3) radionuclides was attained at pH 10.0 (87.6 and 85.6% for 137 Cs + and 85 Sr 2+ , respectively) as indicated by Fig. 1(a) and (b). While the maximum percent uptake of 131 Iwas obtained at pH 1.0 (82.7%) as exposed by Fig. 1(c). Hence, optimum pH was favorite at pH 10.0 for Cs + and Sr 2+ , while for I -, the optimum pH was favored at pH 1.0. Fig. 1

Equilibrium-isotherm-studies
For the optimizing design of a sorption system, itis essential to set the most proper correlations of equilibrium outcomes. The isotherm remains considerable from theoretical and experimental points of view. The knowledge of isotherm nature and its parameters makes it probable to calculate the equilibrium quantity sorbed of adsorbate concentrations outside those employed in this investigation, particularly more diluted ones; moreover, it is needful to scale-up and design the sorption equipment. The adsorption isothermof 137 Cs, 85 Sr, and 131 I radionuclides using GAC is displayed in Fig. 3.

Langmuir model
Langmuir isotherm proposed that all the sites have the same energy and a monolayer ion coverage over a homogeneous surface [19]. Langmuir equation is illustrated as follows.
where Langmuir constants Q and b describe the capacity, mg/g, and sorption energy, According to these values, the sorption onto GAC follows the order Cs + > Sr 2+ >Iwhich is agreed with the order gained from the above studies.  Table 1 3

.5. Freundlich isotherm model
The experimental outcomes can be fitted over a large scale of concentrations by Freundlich description. It provides an expression including the active sites energy and their distribution, and the surface heterogeneity. Freundlich equation is formatted as follows. e e 1 log q = log C + log k n where k and n refer to the capacity and adsorption intensity, qe andCe are defined above. The k and n values are enumerated from intercept and slope, respectively, of log qe against log Ce plot as exposed in Fig. 5. The outcomes illustrated that 1/n<1 for all ions as represented by   Table 2 3.6. Comparison of sorption-capacity of Cs + , Sr 2+ and Ionto different sorbents The monolayer-capacity of Cs + , Sr 2+ , and Iby GAC was compared withother sorbents gained in the literature; the outcomes are stated in Table3. GAC has much higher sorption for the three ions than several materials. It denotes that GAC is recommended to be apromising adsorbent for the capture of Cs + , Sr 2+ , and Ifrom the aqueous phase. Table 3 3

.7. Real application study
The qualification of GAC adsorbent for decontaminating solutions containing 137 Cs, 85 Sr, and 131 I radionuclides was tested. The GAC was applied effectively in the capture of Both phases were detached and the filtrate was radiometrically analyzed. Employment of GAC has exposed that 137 Cs, 85