Description of Ophiogomphus phantoani sp. nov. (Holotype ♂: Dak Re Commune, Song Thanh National Park, Quang Nam Province, central Vietnam, 15.5080 N, 107.4720 E, altitude 1105 m) is based on both sexes. Three other species of Ophiogomphus in Vietnam are recorded with description of the female O. minimus Karube, 2014 for the first time. Keys to males and females of four species from Vietnam are also provided.

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Coeliccia natgeo sp. nov. is described from central Vietnam (holotype ♂, 18.2708 N, 105.3431 E, Khe Nhop, Vu Quang National Park, Ha Tinh Province, deposited in the Zoological Collection of Duy Tan University, Da Nang City, Vietnam). The new species differs from other members of the hayashii-group by males lacking antehumeral stripes, spots and pruinosity on the synthorax and female having the central part of the posterior pronotal lobe reduced to a small projection. Keys to the males and females of the hayashii-group are provided.

Eight species of the genus Prodasineura Cowley, 1934 are recorded from Vietnam, including two newly described species: Prodasineura lancastrei sp. nov. and P. kong sp. nov. from north and central Vietnam. All species recorded are illustrated, including figures of morphological structures, and distribution maps and keys to the males and females are provided. We consider the record of P. laidlawi (Forster in Laidlaw, 1907) a misidentification and exclude it here.

Nowadays, wastewater from various industries contains a large number of harmful heavy metals and coloring agents, which have to be removed to restore the quality of the environment. In this study, the removal of nickel ions (Ni 2+ ) and methylene blue (MB) from the aqueous solution using steel slag as a low cost adsorbent was investigated. The chemical and mineralogical compositions, as well as the surface area of slag, were analyzed by using X-ray fluorescence spectroscopy, X-ray diffraction, and the Brunauer-Emmett-Teller method (BET). The effect of several important parameters such as contact time, adsorbent dose, pH, temperature, and initial adsorbate concentration on the adsorption process was studied systematically by batch experiments. The adsorption data were well correlated with the Langmuir isotherm model by all samples. The maximum adsorption capacity of the raw slag samples was 36.49 mg/g for Ni 2+ and increased from 0.68 to 1.98 mg/g for MB after being acid-activated. The determined thermodynamic parameters indicate that the adsorption of Ni 2+ and MB on steel slag is spontaneous in nature, endothermic (for Ni 2+ ), and exothermic (for MB).