O R I G I N A L A R T I C L E Environment
Shrimp shell as a potential sorbent for removal
of arsenic from aqueous solution
Ming-Chao LinÆ Chung-Min Liao Æ Ying-Cheng Chen
Received: 6 September 2007 / Accepted: 3 September 2008 / Published online: 28 January 2009 Ó The Japanese Society of Fisheries Science 2009
Abstract This study examined the ability of shrimp shell to remove arsenic (As) from aqueous solutions. The shells of two species of shrimp, black tiger shrimp Penaeus monodon and white shrimp Litopenaeus vannamei, were chosen to be the sorbents. Laboratory exposure experi-ments estimated uptake and depuration rate constants (i.e., k1and k2) as well as the bioconcentration factors (BCF) of
the shells of the two shrimps. A first-order one-compart-ment model was presented to describe the uptake kinetics of As in shrimp shell. The resulting k1, k2, and BCF values
of black tiger shrimp were 0.034–1.722 ml/g/day, 0.007– 0.345 g/g/day, and 5.08 ± 1.56 ml/g, while those for white shrimp were 0.053–0.523 ml/g/day, 0.011–0.237 g/g/day, and 3.95 ± 1.88 ml/g, respectively. The sorption capaci-ties of black tiger shrimp shell and white shrimp shell were 1.08 9 10-4–6.66 9 10-3 and 1.04 9 10-4–3.26 9 10-3mmol/g, respectively. The sorption capacity of shrimp shell increased with the initial As concentration in water. Shrimp shell, as a waste material, could be poten-tially used for the removal of As from an aqueous medium. Although the As-removal capacity of shrimp shell was lower than those of natural and chemical sorbents, using shrimp shells as sorbents is less expensive and could increase the additional value of shrimp products.
Keywords Arsenic Shell waste Shrimp Sorption capacity
Introduction
Arsenic (As) is ubiquitous in the environment and is an element notoriously toxic to the general population [1,2]. Classified as a carcinogen, As is emitted into the environ-ment, which poses risks to human health [3]. Contamination of As is globally a major public health issue. The main reason that people are exposed to As is because they come into contact with water, especially groundwater, which contains As. Arsenicosis is a serious disease mainly caused by drinking As-contaminated groundwater [4]. Many studies have shown that As can also be accumulated in seafood [5–7]. Chronic exposure of humans to high con-centrations of arsenic is associated with skin lesions, peripheral vascular disease, hypertension, blackfoot dis-ease, and high risk of cancers [8]. Several epidemiological studies have confirmed that there is an increased risk of cancer in case of exposure to As [9].
A wide range of physical and chemical treatment tech-nologies have been applied for the removal of As from contaminated water, such as coagulation, ultrafiltration, ion exchange, lime softening, adsorption on iron oxides or activated alumina, and reverse osmosis [10]. These are two disadvantages of these treatments. First, there is always a deposited sludge of As compounds. Secondly, the opera-tional costs of these treatments are high. Consequently there is growing interest in using low-cost materials to remove As from water [11]. The adsorption of As has been studied, using a variety of natural materials including sand, clay, kaolinite, bentonite, montmorillonite, goethite, spod-ic, and aquifer materials [12–15]. Among many other kinds
M.-C. Lin (&) Y.-C. Chen
Department of Natural Biotechnology, General Education Center, Graduate Institute of Environmental Management, Nanhua University, Chiayi 622, Taiwan, ROC
e-mail: [email protected] C.-M. Liao
Department of Bioenvironmental Systems Engineering, National Taiwan University, Taipei 106, Taiwan, ROC DOI 10.1007/s12562-008-0044-2
of identified low-cost natural sorbents, chitosan and chitin have the higher sorption capacities for As [16].
Chitosan is slightly soluble at low pHs, and poses problems because it has a tendency to agglomerate or form a gel in aqueous solutions. Chitosan could be obtained from chitin by a deacetylation process with a strong alkaline solution. Chitin is the main component of fungi, algae, molluscs, insects, and crustaceans [17–19]. Recently, chitin was recognized as an excellent sorbent; however, the processes of producing chitin require large amounts of acid and alkaline. Therefore, it is necessary to develop other kinds of techniques for As removal. Raw shrimp shell can be one of the choices among the potential materials [20].
Cultured shrimp is an important commercial crop in Taiwan. The total area of cultivating ponds is up to 9,000 ha. The total amount of supply and marketing of shrimp in Taiwan is 9,000 tons/year. Consumption of shrimp gener-ates enormous amounts of shell waste. According to the stipulation of the Environmental Protection Administration in Taiwan, shrimp shells still remain solid offal at present. If these shrimp shells are thrown aside wantonly, they will cause pollution. In this study, we attempt to utilize shrimp shells as adsorbing materials.
Materials and methods
Among the cultured shrimps, black tiger shrimp (Penaeus monodon) and white shrimp (Litopenaeus vannamei) are two common species in Taiwan. The shrimp shells of these two species were chosen to obtain basic information on the use of shrimp shells for As removal. A total of 540 indi-viduals of shrimp (commercial size, 20 ± 4 g wet weight, 15 ± 1 cm length), 270 individuals of black tiger shrimp and 270 individuals of white shrimp, were obtained from fish markets in Chiayi City, Taiwan, and placed on ice during transfer to the laboratory. The first shell from the abdominal segment of the shrimp was removed from its body and collected for laboratory exposure experiments. The shells were cleaned, rinsed, and then soaked in 1% NaOH at room temperature overnight to remove the bulk of the protein. After being rinsed with distilled water, the shells were dried in a convection oven (DO45, DENG YNG, Taiwan) at 40°C for 3 days.
Prior to the exposure experiment, the shells were placed into the oven at 105 ± 3°C for 1 day and dried to constant weight. The dry weight (g) of each shell was measured using an electronic balance (GT410, OHAUS, USA). Each dried shell was fixed between two glass slides and then photographed using a digital camera (DSC-N1, Sony, Japan). The photograph was traced and the surface area (cm2) of shell was measured using a digitizer (Lab Visions LV-1, Bo-Seng, Taiwan) connected to a computer. The
specific surface area (cm2/g) of shell was calculated by dividing the surface area (cm2) by the dry weight (g). The level of chitin in the shell was analyzed following the method of Juang et al. [21]. The chitin was obtained by immersing the shell in 5 wt.% NaOH for 18 h and later in 5 wt.% HCl (with a weight ratio of 1:10 for shell to solu-tion) for 18 h.
Adsorption assays were conducted to determine the sorption capacities of the shells of the two shrimps. Equi-librium adsorption experiments were performed with varied As concentrations for a period of 14 days. The sorption capacity was examined by exposing the shells to the solutions with As concentrations of 0, 1, 2, 5, 10, 20, 40, 60, 80, and 100 mg/l, prepared from arsenite (Na3AsO3) at room temperature (25 ± 1°C). An amount of
270 dried shells from the same species was separated evenly into ten flasks, each with a volume of 500 ml; thus, every flask contained 27 pieces of shells. One by one the flasks were filled with 400 ml As solutions in the different concentrations as mentioned above. Three pieces of shells were taken from each flask at 0, 3, 6, 12, 24, 48, 96, 168, and 336 h and kept at -20°C before analysis. During the experiments, water samples were taken every 12 h from each flask, acidified by adding 5 ml 1 N HNO3, and stored
for analysis of As concentration. The pH values of solu-tions under various As concentrasolu-tions were measured. After sampling, the water in flasks was renewed immedi-ately to maintain the As concentration. The experiment was repeated twice.
Samples were sent to the Super Micro Mass Research and Technology Center, Cheng Shiu Institute of Technol-ogy for analysis of total As. The shells were dehydrated in a dryer (40°C) for 96 h and ground into powder. Aliquots of dry shell powder weighing 0.5 g were placed into a 250-ml beaker. Nitric acid (65% 10 250-ml) was added and then covered with a glass for an overnight digestion.
After the initial digestion, the beaker covered with a watch glass was heated with a water bath at 70–80°C for 2–4 h to reduce the total volume to 1–2 ml. This volume of solution was transferred to a volumetric flask (50 ml). A mount of 5 ml 0.01 N HNO3was used to rinse the watch
glass. The rinsed solution was added to the flask. The flask was then filled with 0.01 N HNO3to make a 50 ml final
solution. After filtration, this 50 ml solution was trans-ferred to test tubes for As analysis. Arsenic analysis was carried out by using an Agilent 7,500a ICP-MS. Analytical quality control was achieved by digesting and analyzing identical amounts of rehydrated (90% H2O) standard
ref-erence materials (DORM-2, Dogfish Liver-2-organic matrix, NRC-CNRC, Canada). Recovery rates ranged from 95% to 97%.
The sorption capacity of shrimp shell can be calculated based on the following equation [19]:
Q¼ Cw;mðt ¼ 0Þ Cw;mðtÞ
V
m ; ð1Þ
where Q is sorption capacity (mol/g); Cw,m(t = 0) is the
initial concentration of pollutant in water (mol/ml); Cw,m(t)
is the residual concentration of pollutant in water (mol/ml) at time t; V is the volume of water (ml) and m is the weight of sorbent (g).
Since Cw,m(t = 0) - Cw,m(t) = (n0- n1)/MV, where
n0is the initial weight of pollutant in water (g), n1is the
residual weight of pollutant in water (g) and M is the molecular weight of pollutant, Eq.1can be modified as: Q¼ n0n1 MV V m ¼ n0n1 m M : ð2Þ
Under the equilibrium status, the amount of pollutant adsorbed by the sorbent is equal to the amount of pollutant reduced in water (Cs(t = ?) = (n0- n1)/m), Eq.2can be
rewritten as: Q¼Csðt ¼ 1Þ
M ; ð3Þ
where Cs(t = ?) is the equilibrium level of pollutant in
sorbent (g/g).
The equilibrium concentrations of these shells under varied pollutant concentrations in water were calculated using an iterative, nonlinear, least-squares curve-fitting technique, based on a first-order one-compartment model [22]:
CsðtÞ ¼ Csðt ¼ 0Þ þ Cwðt ¼ 1Þ
k1
k2
ð1 ek2tÞ; ð4Þ where Cs(t) is the pollutant level in sorbent (lg/ml) at time
t; Cs(t) is the initial pollutant level in sorbent (g/g);
Cw(t = ?) is the equilibrium concentration of pollutant in
water (g/ml); k1is the uptake rate constant (ml/g/day) and
k2is the depuration rate constant (g/g/day). The constants
k1and k2, indicate the inflow and outflow rates between the
sorbent and its ambient water, respectively.
When under the equilibrium condition in which Cs(t = 0) = 0 and t = ?, then Eq.4can be calculated as
Csðt ¼ 1Þ ¼ Cwðt ¼ 1Þ
k1
k2
; ð5Þ
and the sorption capacity (Q) can be calculated as Q¼Csðt ¼ 1Þ M ¼ Cwðt ¼ 1Þ M k1 k2 : ð6Þ
When steady-state pollutant concentrations of sorbent are attained, the equilibrium bioconcentration factor (BCF) of the sorbent can be calculated from the ratio of the pollutant concentration in sorbent to that in water.
The BCF can also be calculated from the ratio of the uptake rate constant to the depuration rate constant as
BCF¼Csðt ¼ 1Þ Cwðt ¼ 1Þ
¼k1 k2
; ð7Þ
where BCF (ml/g) is the equilibrium bioconcentration factor of the sorbent. And therefore
Q¼Cwðt ¼ 1Þ
M BCF: ð8Þ
The curve fitting in this study was performed using the nonlinear regression option of the StatisticaÒ software (StatSoft, Tulsa, OK, USA) to simulate the values of k1and
k2, based on Eq.4 and the data of Cs(t = ?) and
Cw(t = ?). The BCF value was then calculated by
dividing k1by k2.
Results
The wet weight, dry weight, and length of black tiger shrimp were 21.51 ± 1.64 g, 4.89 ± 0.42 g, and 15.49 ± 0.55 cm, respectively, while those of white shrimp were 17.93 ± 1.58 g, 4.34 ± 0.39 g, and 14.61 ± 0.36 cm, respectively. The wet weight, dry weight, surface area, and specific surface area of shells of black tiger shrimp were 0.126 ± 0.015 g, 0.065 ± 0.012 g, 7.08 ± 1.31 cm2, and 0.011 m2/g, respectively, while those of shells of white shrimp were 0.093 ± 0.015 g, 0.050 ± 0.006 g, 11.59 ± 1.38 cm2, and 0.023 m2/g, respectively. The shell of black tiger shrimp contained 46.09 ± 11.39% water of the total wet weight. The water in the shell of white shrimp was 44.62 ± 6.40% of total wet weight. The protein and chitin in the shell of black tiger were 22.83 ± 5.64% and 32.13 ± 7.94% of dry weight, respectively, while those in the shell of white shrimp were 25.85 ± 3.71% and 41.41 ± 5.94% dry weight.
Analysis of variance (ANOVA) on the resulting data showed that the As concentration in water did not change significantly during the exposure experiments for black tiger shrimp (F = 903.56, n = 90, P \ 0.05) or for white shrimp (F = 1010.71, n = 90, P \ 0.05) (Tables1 and 2). The As level in shrimp shell, however, increased significantly compared with the initial As level. The As level in the shell of black tiger shrimp showed a sig-nificant positive relation to exposure time (Fig. 1). A similar phenomenon was also found in white shrimp (Fig. 2). The uptake of As in shrimp shell was higher initially, reducing with increasing time, and reached an equilibrium level. The shrimp shells exposed to the solutions with As concentrations 20–100 mg/l reached an equilibrium status within 96 h, while they did not reach an equilibrium status under 0–10 mg/l during the 96-h period. The pH values of the solutions remained between 6.03 and 7.02.
Table 1 The values of pH, the As level in the shell of black tiger shrimp, and the actual As concentration of water at different exposure time under various As con centrations (C w ) Cw (mg/l ) pH Time (h) 0 3 6 1 2 2 4 4 8 9 6 168 336 0 6.98 ± 0.17 0.23 ± 0.07 a 0.28 ± 0.20 a 0.22 ± 0.07 a 0.23 ± 0.16 a 0.17 ± 0.07 a 0.21 ± 0.06 a 0.31 ± 0.41 a 0.30 ± 0.30 a 0.25 ± 0.28 a 0.04 ± 0.02 b 0.02 ± 0.01 b 0.01 ± 0.01 b 0.00 ± 0.00 b 0.02 ± 0.02 b 0.04 ± 0.01 b 0.02 ± 0.01 b 0.01 ± 0.00 b 0.01 ± 0.00 b 1 6.03 ± 0.14 0.78 ± 0.04 a 0.46 ± 0.04 a 0.58 ± 0.07 a 1.62 ± 1.26 a 3.38 ± 2.28 a 4.32 ± 1.53 a 6.56 ± 3.96 a 5.30 ± 1.49 a 9.35 ± 3.02 a 1.16 ± 0.90 b 0.96 ± 0.28 b 1.14 ± 0.35 b 0.79 ± 0.59 b 1.02 ± 0.30 b 1.26 ± 0.61 b 0.87 ± 0.50 b 0.98 ± 0.35 b 1.29 ± 0.54 b 2 6.03 ± 0.25 0.52 ± 0.27 a 0.69 ± 0.34 a 1.06 ± 0.49 a 1.25 ± 0.47 a 1.90 ± 0.94 a 3.92 ± 1.41 a 4.04 ± 0.67 a 5.62 ± 1.21 a 8.90 ± 0.65 a 1.84 ± 0.81 b 2.10 ± 0.47 b 2.12 ± 1.54 b 1.78 ± 0.38 b 1.89 ± 0.68 b 2.24 ± 0.54 b 2.12 ± 1.08 b 1.74 ± 1.01 b 2.19 ± 0.85 b 5 6.30 ± 0.27 0.95 ± 0.27 a 1.56 ± 0.19 a 1.59 ± 0.51 a 4.26 ± 1.36 a 6.01 ± 3.42 a 8.42 ± 3.02 a 10.34 ± 3.45 a 13.2 8 ± 19.24 a 19.24 ± 8.02 a 5.26 ± 2.78 b 5.11 ± 0.97 b 4.88 ± 2.00 b 4.80 ± 1.54 b 5.31 ± 1.47 b 5.20 ± 0.80 b 4.67 ± 2.72 b 5.48 ± 1.56 b 5.05 ± 1.12 b 10 6.17 ± 0.11 1.01 ± 0.62 a 5.10 ± 2.11 a 5.04 ± 2.45 a 17.4 0 ± 3.81 a 13.09 ± 3.55 a 25.4 0 ± 6.61 a 30.38 ± 18.7 4 a 41.0 7 ± 7.48 a 53.68 ± 14.9 4 a 9.68 ± 6.50 b 9.74 ± 6.11 b 11.4 5 ± 3.40 b 11.6 4 ± 3.84 b 9.15 ± 4.57 b 9.48 ± 3.95 b 10.85 ± 6.96 b 11.1 5 ± 3.87 b 10.33 ± 4.57 b 20 6.15 ± 0.09 0.10 ± 0.04 a 23.0 7 ± 9.21 a 34.8 7 ± 13.25 a 55.8 3 ± 27.1 4 a 65.33 ± 18.2 9 a 71.1 7 ± 20.0 1 a 77.50 ± 11.8 2 a 76.7 5 ± 19.64 a 77.08 ± 17.9 8 a 21.45 ± 7.32 b 20.1 0 ± 3.2 b 19.4 1 ± 5.52 b 21.3 7 ± 8.77 b 18.27 ± 8.45 b 19.7 8 ± 5.84 b 22.50 ± 2.64 b 17.8 7 ± 8.40 b 19.78 ± 11.9 2 b 40 6.37 ± 0.18 0.80 ± 0.55 a 41.0 0 ± 25.7 1 a 68.2 6 ± 29.86 a 89.4 7 ± 14.0 5 a 93.66 ± 39.5 1 a 115. 83 ± 32.0 7 a 123.85 ± 50.2 3 a 120. 98 ± 44.91 a 118. 02 ± 88.4 3 a 43.54 ± 13.0 8 b 38.6 9 ± 4.98 b 39.7 1 ± 11.91 b 37.5 8 ± 14.0 7 b 44.45 ± 19.8 2 b 41.6 9 ± 18.1 4 b 38.10 ± 21.1 8 b 40.8 4 ± 12.05 b 45.52 ± 19.8 2 b 60 6.41 ± 0.18 0.39 ± 0.17 a 194. 50 ± 89.0 7 a 276.67 ± 92.84 a 347. 67 ± 118. 24 a 407.80 ± 139. 99 a 352. 42 ± 210. 69 a 452.33 ± 161. 02 a 386. 06 ± 143.5 a 400. 38 ± 256. 12 a 64.85 ± 21.0 4 b 58.5 4 ± 15.6 7 b 60.8 8 ± 21.41 b 65.7 8 ± 12.5 1 b 57.57 ± 20.3 1 b 54.3 2 ± 21.0 1 b 59.70 ± 4.89 b 62.8 8 ± 15.87 b 64.32 ± 27.1 5 b 80 6.70 ± 0.53 0.64 ± 0.11 a 266. 67 ± 107. 72 a 288.33 ± 207.58 a 399. 67 ± 179. 61 a 269.08 ± 126. 89 a 447. 78 ± 229. 81 a 628.33 ± 259. 91 a 490. 56 ± 389.01 a 408. 92 ± 210. 67 a 78.39 ± 32.0 1 b 81.8 9 ± 12.8 4 b 85.7 6 ± 8.04 b 76.8 9 ± 23.3 2 b 73.04 ± 19.2 5 b 87.0 8 ± 8.10 b 77.56 ± 17.1 4 b 84.3 7 ± 21.01 b 80.87 ± 24.1 5 b 100 6.60 ± 0.08 0.34 ± 0.31 a 337. 67 ± 21.8 9 a 407.33 ± 245.68 a 517. 33 ± 184. 95 a 529.67 ± 410. 51 a 430. 56 ± 267. 58 a 586.57 ± 127. 13 a 510. 58 ± 294.51 a 490. 01 ± 263. 47 a 111. 56 ± 15.1 0 b 107. 84 ± 11.4 4 b 104.51 ± 11.47 b 94.4 1 ± 25.5 6 b 89.45 ± 27.4 0 b 104. 57 ± 28.4 5 b 119.10 ± 27.4 5 b 94.4 7 ± 24.75 b 97.45 ± 25.3 6 b a The As level in the shell (l g/g ) b The actual As conc entration in water (mg/l )
Table 2 The values of pH, the As level in the shell of white shrimp, and the actual As concentration in water measured at different exposure time under various As concentrations (C w ) Cw (mg/l) pH Time (h) 0361 2 2 4 4 8 9 6 1 6 8 3 3 6 0 7.04 ± 0.21 0.17 ± 0.08 a 0.21 ± 0.04 a 0.24 ± 0.17 a 0.20 ± 0.11 a 0.15 ± 0.04 a 0.21 ± 0.10 a 0.25 ± 0.17 a 0.17 ± 0.04 a 0.21 ± 0.13 a 0.01 ± 0.01 b 0.02 ± 0.01 b 0.02 ± 0.01 b 0.02 ± 0.00 b 0.05 ± 0.01 b 0.10 ± 0.07 b 0.05 ± 0.01 b 0.02 ± 0.01 b 0.01 ± 0.01 b 1 6.18 ± 0.50 0.26 ± 0.05 a 0.65 ± 0.23 a 0.95 ± 0.30 a 1.90 ± 1.08 a 2.84 ± 0.65 a 2.91 ± 1.40 a 4.38 ± 1.27 a 7.41 ± 1.67 a 7.79 ± 0.65 a 1.08 ± 0.77 b 1.05 ± 0.39 b 1.22 ± 0.13 b 1.20 ± 0.80 b 1.13 ± 0.41 b 0.99 ± 0.62 b 0.94 ± 0.36 b 1.09 ± 0.18 b 0.95 ± 0.57 b 2 6.22 ± 0.50 0.37 ± 0.06 a 1.79 ± 0.15 a 2.65 ± 0.24 a 3.54 ± 0.21 a 5.57 ± 0.64 a 7.27 ± 1.03 a 9.44 ± 1.77 a 11.4 7 ± 3.76 a 13.08 ± 2.83 a 2.04 ± 0.78 b 2.06 ± 0.65 b 1.86 ± 0.46 b 2.13 ± 1.00 b 2.10 ± 1.11 b 1.81 ± 1.24 b 1.88 ± 0.51 b 2.03 ± 0.78 b 1.94 ± 0.66 b 5 6.27 ± 0.49 0.98 ± 0.62 a 1.62 ± 1.11 a 1.58 ± 1.45 a 4.18 ± 3.81 a 5.15 ± 3.55 10.54 ± 6.61 a 12.6 0 ± 8.74 15.8 8 ± 7.48 a 19.55 ± 15.4 1 a 4.85 ± 1.48 b 5.27 ± 1.55 b 4.10 ± 1.04 b 4.91 ± 2.36 b 4.97 ± 0.98 b 5.17 ± 1.92 b 5.30 ± 2.44 b 4.86 ± 1.34 b 4.88 ± 2.31 b 10 6.37 ± 0.50 0.24 ± 0.04 a 6.79 ± 0.51 a 20.9 1 ± 0.48 a 19.9 9 ± 1.74 a 27.9 0 ± 4.24 a 25.38 ± 1.83 a 35.1 5 ± 4.11 42.7 8 ± 6.49 a 50.09 ± 29.0 5 a 11.9 4 ± 3.94 b 12.6 4 ± 7.11 b 9.48 ± 4.41 b 9.84 ± 7.20 b 10.6 8 ± 3.20 b 11.04 ± 0.54 b 9.76 ± 6.10 b 10.4 5 ± 3.65 b 10.78 ± 6.66 b 20 6.42 ± 0.55 0.10 ± 0.04 a 23.4 0 ± 14.3 3 a 48.1 0 ± 15.5 8 a 60.2 7 ± 35.0 1 a 79.6 5 ± 33.65 a 63.91 ± 18.7 9 a 73.3 1 ± 31.4 2 a 75.6 9 ± 23.21 a 70.36 ± 7.35 a 19.7 8 ± 8.91 b 18.9 8 ± 6.65 b 21.8 7 ± 5.55 b 19.4 4 ± 8.26 b 23.6 6 ± 10.63 b 21.65 ± 8.12 b 23.1 5 ± 3.94 b 19.5 6 ± 9.49 b 19.97 ± 8.99 b 40 6.51 ± 0.62 0.28 ± 0.55 a 31.4 8 ± 22.6 4 a 66.5 1 ± 29.6 5 a 88.6 9 ± 24.0 4 a 95.4 9 ± 4.68 a 104.98 ± 33.1 5 a 106. 32 ± 48.2 7 a 105. 66 ± 72.08 a 97.08 ± 27.8 0 a 46.0 2 ± 15.6 2 b 40.2 0 ± 13.6 4 b 35.6 5 ± 12.2 5 b 41.3 6 ± 18.3 9 b 38.9 1 ± 11.25 b 39.65 ± 11.9 4 b 42.0 1 ± 20.0 3 b 44.6 2 ± 13.65 b 37.15 ± 7.95 b 60 6.70 ± 0.53 0.37 ± 0.09 a 106. 34 ± 49.5 8 a 128. 45 ± 65.2 5 a 149. 77 ± 55.2 5 a 169. 42 ± 98.56 a 160.26 ± 35.9 8 a 165. 95 ± 71.1 5 a 152. 94 ± 26.35 a 171.65 ± 105. 21 a 59.6 5 ± 11.0 3 b 57.3 2 ± 20.3 0 b 64.3 8 ± 1.65 b 61.2 5 ± 22.0 1 b 62.3 5 ± 5.14 b 59.24 ± 16.2 5 b 54.9 5 ± 17.6 8 b 58.4 7 ± 26.23 b 62.37 ± 5.64 b 80 6.85 ± 0.69 0.29 ± 0.15 a 114. 67 ± 80.1 5 a 118. 54 ± 38.9 4 a 147. 67 ± 88.5 1 a 167. 21 ± 100.26 a 180.95 ± 29.1 5 a 194. 77 ± 68.2 4 a 215. 32 ± 118.54 a 189.54 ± 36.8 7 a 85.2 5 ± 9.25 b 78.0 1 ± 18.2 5 b 74.6 9 ± 12.1 5 b 88.3 5 ± 21.3 2 b 81.2 7 ± 21.45 b 79.62 ± 15.2 4 b 88.4 7 ± 15.3 0 b 75.6 8 ± 26.82 b 84.26 ± 13.1 5 b 100 7.02 ± 0.92 0.32 ± 0.18 a 152. 67 ± 97.2 5 a 168. 64 ± 78.2 4 a 233. 54 ± 139. 54 a 229. 61 ± 120.49 a 226.67 ± 103. 25 a 270. 65 ± 127. 13 a 278. 69 ± 88.95 a 286.91 ± 201. 63 a 106. 54 ± 12.5 6 b 98.6 7 ± 3.64 b 113. 02 ± 14.0 9 b 101. 26 ± 23.6 4 b 98.0 4 ± 12.37 b 99.57 ± 21.7 5 b 118. 62 ± 22.2 7 b 111. 14 ± 16.98 b 94.25 ± 23.1 0 b a The A s level in the shell (l g/g ) b The actual As conce ntratio n in w ater (mg/l )
The kinetic uptake data were successfully modeled using the first-order one-compartment model incorporating nonlinear adsorption, with high R2 values of 0.67–0.99. The k1, k2, and BCF values of black tiger shrimp were
0.034–1.722 ml/g/day, 0.007–0.345 g/g/day, and 5.08 ± 1.56 ml/g (Table3), while those of white shrimp were 0.053–0.523 ml/g/day, 0.011–0.237 g/g/day, and 3.95 ± 1.88 ml/g, respectively (Table4). The values of BCF showed that the shrimp shells can accumulate a high level of waterborne As. The sorption capacities of these shells were calculated based on Eq.5. The sorption capacities of black tiger shrimp shell and white shrimp shell were 1.08 9 10-4–6.66 9 10-3, and 1.04 9 10-4–3.26 9 10-3mmol/g, respectively (Table5). ANOVA of the resulting data showed that the sorption capacities of the shell of black tiger shrimp did not significantly differ from those of white shrimp (F = 1.50, n = 18, P [ 0.05).
A time needed to reach equilibrium status, i.e., equi-librium time, of longer than 336 h was calculated using the first-order one-compartment model (Eq.4) and the
nonlinear regression option of the StatisticaÒsoftware. The resulting data for equilibrium time under various As con-centrations are shown in Fig.3. The equilibrium time
0 100 200 300 400 500 600 700 0 10 20 30 40 50 60 70 80 90 100 Time (h)
As level in shrimp shell (
µ g/g) 0 10 20 30 40 50 60 0 50 100 150 200 250 300 350 Time (h)
As level in shrimp shell (
µ g/g) 100 mg/l 80 mg/l 60 mg/l 20 mg/l 40 mg/l 10 mg/l 5 mg/l 1 mg/l 2 mg/l a b
Fig. 1 Plot of the relation between the As level of black tiger shrimp
shell and the exposure time under various As concentrations in water (a 1–10 mg/l; b 20–100 mg/l) 0 10 20 30 40 50 0 50 100 150 200 250 300 350 Time (h) As le vel in shri mp shel l (µ g/ g) 0 50 100 150 200 250 300 0 10 20 30 40 50 60 70 80 90 100 Time (h) As le vel in shri mp shel l (µ g/ g) 10 mg/l 5 mg/l 1 mg/l 2 mg/l 100 mg/l 60 mg/l 80 mg/l 40 mg/l 20 mg/l a b
Fig. 2 Plot of the relation between the As level of white shrimp
shell and the exposure time under various As concentrations in water (a 1–10 mg/l; b 20–100 mg/l)
Table 3 Values of uptake rate constant (k1), depuration rate constant
(k2), BCF, and R2 of black tiger shrimp shell under various As
concentrations
Cw(mg/l) k1(ml/g per day) k2(g/g per day) BCF (ml/g) R2
1 0.129 0.016 8.06 0.98 2 0.034 0.007 4.86 0.97 5 0.040 0.011 4.00 0.99 10 0.075 0.015 5.00 0.90 20 0.411 0.112 3.67 0.99 40 0.366 0.125 2.93 0.91 60 1.310 0.197 6.65 0.99 80 1.168 0.209 5.59 0.67 100 1.722 0.345 4.99 0.88 Mean 5.08 ± 1.56
showed a significant negative relation to the initial As concentration in water (Fig. 3). The shrimp shells of the two species had higher equilibrium As levels under higher initial As concentrations in water (Fig.4). This showed that the shrimp shells have different sorption capacities under various initial As concentrations in ambient water. When the initial As concentration in water is higher, the shrimp shell showed a higher sorption capacity.
Figures5and6show that the As levels in shrimp shell changed under various As exposure concentrations in water at different times. Both the data obtained from the actual measurements (Fig.5) and the data obtained based on the As level in shrimp shell (Fig.6) showed that the decrease of As concentration in water could be neglected compared with the increase of the As level in shrimp shell.
Discussion
The BCF of the shell of black tiger shrimp did not sig-nificantly differ from that of white shrimp. The sorption capacity also did not show a significant difference between the two shrimp species. The first-order one-compartment model (Eq.4) was successfully fitted to the uptake curve of As levels in shrimp shell. The high R2 values (generally higher than 0.9) indicated that this model is appropriate to predict the As level in shrimp shell at different time points under a certain concentration. In this study, the exposure experiment was conducted under a condition of high As concentrations (1–100 mg/l) in a large quantity of water (400 ml) and low quantity of shrimp shells (approximately
Table 4 Values of uptake rate constant (k1), depuration rate constant
(k2), BCF, and R2 of white shrimp shell under various As
concentrations
Cw(mg/l) k1(ml/g per day) k2(g/g per day) BCF (ml/g) R2
1 0.086 0.011 7.82 0.96 2 0.134 0.022 6.09 0.95 5 0.053 0.014 3.79 0.97 10 0.207 0.051 4.06 0.82 20 0.445 0.118 3.77 0.91 40 0.399 0.152 2.63 0.99 60 0.379 0.139 2.73 0.99 80 0.523 0.237 2.21 0.99 100 0.501 0.205 2.44 0.95 Mean 3.95 ± 1.88
Table 5 Sorption capacities of the shells of black tiger shrimp and
white shrimp under various As concentration in water As concentration
(mg/l)
Black tiger shrimp (mmol/g) White shrimp (mmol/g) 1 1.08 9 10-4 1.04 9 10-4 2 1.30 9 10-4 2.47 9 10-4 5 2.67 9 10-4 2.53 9 10-4 10 6.67 9 10-4 5.42 9 10-4 20 9.80 9 10-4 1.01 9 10-4 40 1.56 9 10-3 1.40 9 10-3 60 5.33 9 10-3 2.18 9 10-3 80 5.97 9 10-3 2.36 9 10-3 100 6.66 9 10-3 3.26 9 10-3 0 100 200 300 400 500 600 700 0 20 40 60 80 100 120 Initial As concentration in water (mg/l)
E q u ilib ri u m tim e (h )
Fig. 3 Plot of the relation between the equilibrium time for shells of
the two shrimp species, black tiger shrimp (filled triangle) and white shrimp (open circle), and the As concentration in water
Cs(t = ) Cs(t = ) = 5.26 Cw R2 = 0.946 = 2.46 Cw R2 = 0.971 0 100 200 300 400 500 600 0 20 40 60 80 100 120 As concentration in water (mg/l) E q u ili briu m A s lev e l in shrim p shell ( µ g/g)
Fig. 4 Plot of the relation between the equilibrium As level in shells
of the two shrimp species, black tiger shrimp (filled triangle) and
white shrimp (open circle), and the As concentration in water. Csis
the equilibrium level of pollutant in sorbent (lg/g); Cw(t = ?) is the
0.05 g dry weight per piece). In this case the decrease of As in water can be neglected because of the large quantity of water and high As concentrations in water, but the increase of As in shrimp shells cannot be neglected because of the small quantity of shrimp shells. Under this condition of high pollutant concentration in a large quantity of water and small quantity of sorbents, Eq.8can simply be used to calculate the sorption capacity of sorbent, based on the equilibrium concentration of pollutant in water, the molecular weight of pollutant, and the BCF of the sorbent. The first-order one-compartment model is usually used to simulate the bioaccumulation, including bioconcentra-tion and/or biomagnificabioconcentra-tion, of pollutants in living organisms. The uptake rate (k1) and depuration rate (k2)
could be influenced by the active intake/elimination behaviors of organisms and/or be influenced passively by the concentration gradients. Since shrimp shell is a non-living matter, k1and k2are only passively influenced by the
concentration gradients. Under this situation, the first-order one-compartment model should still be suitable to simulate the As level in shrimp shell based on k1 and k2 of the
shrimp shell.
Dambies et al. [17] have shown that the sorption capacity of raw chitosan beads for As(III) removal remained constant, independently of the pH. The influence of pH change (6.03–7.02) on the sorption capacity of shrimp shell in this study can be neglected. Many previous studies have shown that pollutant uptake in a sorbent decreases with increasing time. Evans et al. [23] indicated that, following the stage of initial adsorption at the surface of sorbent, the uptake rate is controlled by intraparticle diffusion in the sorbent. In our study, we found that the uptake rate is influenced simultaneously by k1and k2of the
sorbent. Normally the sorption capacity of a sorbent should be a constant. Our study showed that the sorption capacity can be influenced by the initial pollutant concentration in water. Shrimp shells exposed to higher As concentrations can reach equilibrium status within a shorter period, indicting that the shrimp shells have higher sorption capacities under higher As concentrations in water. The exposure experiments in this study took place in static water without any agitation. Under this condition, a greater concentration gradient on the interface between water and 3 h6 h 3 h 6 h 12 h 96 h 6 h 96 h 336 h 3 h 6 h 48 h 96 h 336 h 0 h 336 h 0 h 336 h 0 h 336 h 0 h 12 h 24 h48 h 96 h 168 h 336 h 0 h 24 h 48 h168 h 336 h 0 h 3 h 24 h 48 h 168 h 0 h 12 h 24 h 168 h 0 100 200 300 400 500 600 700 0 20 40 60 80 100 120 140 As concentration in water (mg/l)
As level in shrimp shell
( µ g/g) 3 h 6 h 3 h 6 h 24 h 3 h 6 h 336 h 0 h 336 h 0 h 336 h 0 h 336 h 0 h 12 h 24 h 48 h 96 h 168 h 336 h 0 h 12 h 48 h 96 h 168 h 336 h 0 h 3 h 6 h 24 h 48 h 96 h 168 h 336 h 0 h 12 h 24 h 48 h 96 h 168 h 0 50 100 150 200 250 300 350 0 20 40 60 80 100 120 140 As concentration in water (mg/l )
As level in shrimp shell
(µ
g/g)
Black Shrimp
White Shrimp
Fig. 5 Plot of the As level of shrimp shell and the As concentration
in water, based on the actual measurement, at various exposure times. Symbols indicating the original As concentrations in water: filled square 100 mg/l, filled triangle 80 mg/l, filled circle 60 mg/l, filled diamond 40 mg/l, asterisk 20 mg/l, times 10 mg/l, minus 5 mg/l, plus 3 mg/l, hyphen 1 mg/l 3 h 6 h 12 h 48 h 3 h 6 h 12 h 24 h 96 h 6 h 48 h 96 h 168 h 336 h 3 h 6 h 12 h 24 h 48 h 96 h 336 h 0 h 336 h 0 h 336 h 0 h 336 h 0 h 24 h 96 h 168 h 336 h 0 h 48 h168 h 336 h 0 h 3 h 24 h 0 h 168 h 0 100 200 300 400 500 600 700 0 20 40 60 80 100 120 As concentration in water (mg/l)
As level in shrimp shell
( µ g/g) 336 h 3 h 6 h 12 h 48 h 3 h 6 h 96 h 168 h 336 h 3 h 6 h 0 h 336 h 0 h 336 h 0 h 0 h 24 h 96 h 168 h 336 h 0 h 12 h 24 h 48 h 168 h 336 h 0 h 3 h 6 h 24 h 48 h 96 h 0 h 12 h 24 h 48 h 96 h 168 h 336 h 0 50 100 150 200 250 300 350 0 20 40 60 80 100 120 As concentration in water (mg/l)
As level in shrimp shell
(µ
g/g)
Black Shrimp
White Shrimp
Fig. 6 Plot of the As level of shrimp shell and the As concentration
in water, calculated based on the As level of shrimp shell, at various exposure times. Symbols indicating the original As concentrations in water: filled square 100 mg/l, filled triangle 80 mg/l, filled circle 60 mg/l, filled diamond 40 mg/l, asterisk 20 mg/l, times 10 mg/l, minus 5 mg/l, plus 3 mg/l, hyphen 1 mg/l
shrimp shell, which remains undisturbed around the shell, forms a concentration boundary layer (i.e., diffusion layer) between the shell surface and the bulk solution. The higher the As concentration in water, the more it enhances the absorption of As on shrimp shell and the earlier the As level on shrimp shell reaches the equilibrium status.
The BCF of the shell of black tiger shrimp did not significantly differ from that of white shrimp. The sorption capacity also did not show a significant difference between the two shrimp species. Dambies et al. [17] noted that the sorption capacities of activated carbon, activated alumina, activated mineral surfaces, silica, bauxite, coral limestone, and chitin were commonly lower than 0.1–0.2 mmol/g. Among these sorbents, activated carbon, and activated alumina, currently regarded as advantageous sorbents, have sorption capacities of 0.60 and 0.27 mmol/g, respectively [24, 25]. The As-removal capacity of shrimp shell was lower than those of natural and chemical sorbents. A large amount of shrimp shells might be needed to remove a small amount of As from a solution. Besides circulating the solution to stream over the shrimp shells, renewing the shrimp shells after a certain expose period will be helpful to compensate for the low As sorption capacity of shrimp shell. Since shrimp shell is a waste product, the cost of this sorbent may be lower than that of the other mentioned sorbents.
The sorption capacities (1–2 mmol/g for Pb, Cd, Cu, and Cr removal) of pulverized crab shell [19] were higher than the sorption capacity (1.04 9 10-4–6.66 9 10-3mmol/g for As(III) removal) of raw shrimp shell (this study). Compared with the sorption capacities (2.52–6.2 mmol/g for Cu(II) removal and 1.9 9 10-2mmol/g for As(III) removal) of prepared chitosan bead [17, 21] the sorption capacity of raw shrimp shell (this study) was also relatively low. The specific surface areas of raw shrimp shell were 0.011–0.023 m2/g, while those of the crab shell particle and the chitosan bead were 13.35 and 12.50 m2/g, respectively. This difference in specific surface area could be the main reason for the significant difference in sorption capacity. Compared with the chitin level (26.65% dry weight) in crab shell, the two species of shrimp contained higher chitin levels (32.13 ± 7.94% dry weight for black tiger shrimp and 41.41 ± 5.94% dry weight for white shrimp) in the shell. This indicates that shrimp shell can be an ideal material as a potential sorbent for pollutants removal.
High concentrations of As were found in cultured fish from southwest Taiwan since As-contaminated ground-water was used for aquaculture in this area [5–7]. Several studies have been conducted to demonstrate that use of As-contaminated groundwater for aquaculture may cause overexposure of As in fish [5, 6, 26]. Ingestion of As-contaminated fish could result in As accumulation in
inhabitants and lead to adverse health effects [7,27]. Many cultured stocks, such as eel, carp, and shrimp, from this area may also be contaminated by As. Public health experts are concerned since it has been known for years that using groundwater for aquaculture is a common situation in the As-contaminated area in Taiwan [28] but few As-removal techniques have been developed. With a capacity of removing As from aqueous solution, shrimp shells may have the potential to remove As from As-contaminated pond water.
From the results of our study it can be concluded that, even when not much As is removed from the water because of the small amount and low absorption capacity of shrimp shell, the shrimp shell is still helpful to remove As because it is cheap and can therefore be used in large quantities. Because the absorption capacity of raw shrimp shell is rather low, a large amount of shells needs to be used for the removal of As from the aqueous solution. After the shells are chemically satiated by As, the shell scrap may cause environmental problems. Dambies et al. [17] have pro-vided useful information to solve these problems. They have noted that phosphate anion can strongly reduce As sorption, and this property is useful for the desorption of As-saturated sorbents. Phosphoric acid at an intermediary concentration (0.1–0.2 M) was suggested to be used for the recovery of As and the regeneration of the sorbents. It is necessary in further studies to research the suitability of using phosphate acid for the desorption of As from shrimp shell.
The removal of shells from shrimps, as well as other shellfish, crabs, and krill creates a large amount of waste [29]. Using these shell wastes could also increase the additional value of fishery and aquaculture products. It has been shown that the use of shrimp shell for As removal appears to be technically feasible. Shrimp shell, as a natural material, is ideally available with low cost and is environmentally friendly. Moreover, being composed entirely of aquacultural and fishing industry waste, it helps in reduction of waste generation. Because shrimp shells can be easily obtained, employed, and disposed of with little cost, they could be good candidates for adsorption of As in wastewater streams. The use of shrimp shells as sorbents should be investigated as a replacement for cur-rent costly methods of removing heavy metals from solution. The mechanism of As adsorption in shrimp shells should be revealed. To make the As removal more efficient, the use of a significant amount of shells of shrimp shell is required, which implies that more research should be done on the desorption of As from the shells. Furthermore, studies concerning As as well as the other metals adsorbed by shrimp shells need to be undertaken to provide more information on shrimp shells as low-cost sorbents.
Acknowledgments This work was financially supported by the National Science Council of the Republic of China (NSC 93-2313-B-343-001). The authors thank Dr. G.P. Chang-Chien for providing analysis facilities and Miss W.C. Wu for deploying experimental assistance. An earlier draft of this manuscript benefited from the comments of Mr. R. Regout.
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