Fig. 6-5 Scallop (left) and midgut glands (right) detached after processing in boiling water Waste midgut glands, which contain potentially useful compounds such as abundant proteins and docosahexaenoic acid, have been detached and incinerated up to now, since they contain toxic Cd. In this work, the Cd has been extracted using a malic acid solution and then effectively removed with a graft adsorbent.

Fig. 6-6 Characteristics of Cd adsorption with graft adsorbents Amidoxime adsorbent is more profitable for removal of Cd than iminodiacetic acid adsorbent, since the pH of malic acid solution for extracting Cd is 3.3.

Fig. 6-7 Removal of Cd ions with column packing iminodiacetic acid adsorbent Removal of Cd ions was performed at a high space velocity of 230 h-1. Adsorbed Cd can be eluted with hydrochloric acid and the adsorbent column can be regenerated.

Scallops are a specialty product of Aomori prefecture. This prefecture produces one hundred thousand tons of cultured scallops per year. In the process of boiling and packing scallop adductor muscle in cans (Fig. 6-5), 20 thousand tons of wastes, such as midgut glands and genital glands are generated. The waste midgut glands, which contain valuable nutrients such as docosahexaenoic acid, eicosapentaenoic acid, vitamins, and minerals, are currently incinerated, since they contain toxic cadmium (Cd) in the range of 10 to 20 ppm. Radiation-induced graft polymerization can be used to synthesize adsorbents which effectively remove toxic metal ions. A particularly effective metal-adsorbent fiber can be synthesized by radiation processing of a nonwoven fabric with an electron beam. Although Cd impurities are strongly bonded to the denatured proteins of the midgut gland after the process of boiling, the Cd can be extracted by dipping the boiled midgut glands into malic acid, which can be used as food. Iminodiacetic acid adsorbent was more profitable for removal of the Cd than amidoxime adsorbent, since the pH of the malic acid solution used to extract the Cd is 3.3 (Fig. 6-6). Cd ions in the malic acid solution are then removed using as a column packing an iminodiacetic acid adsorbent which has been synthesized using the radiation-induced graft polymerization technique (Fig. 6-7). As a result, the concentration of Cd ions in a volume treated with malic acid which is 120 times the volume of the column could be effectively reduced to below the legal limit of 0.1 ppm, using a flow rate of 230 times the column volume per hour. This project has been adopted by an industrial technology program offered by the New Energy and Industrial Technology Development Organization (NEDO) and has been carried out in cooperation with Hachinohe Technical Laboratory (Aomori Industrial Research Center), Hachinohe National College of Technology, and Aomori Scallop Waste Disposal Cooperative, with the aim of its being put into practical use.

Reference H. Nakai et al., Study of System to Utilize the Waste of Scallop Processing, J. Ion. Exchange, 15(1), 10 (2004) (in Japanese).

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