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The leather tanning industry is well-known for its severe negative impact on the environment. In this industry animal hides are transformed into leather in a succession of several complex stages, consuming high quantities of water and using large amounts of chemicals such as lime, sodium sulfide, ammonium sulfate, sodium chloride, bactericides, vegetable tannins, and chrome salts, etc. Tanneries wastewaters (WW) are mainly characterized by high salinity, high organic loading, and specific pollutants, such as sulfide and chromium [1]. The tanning processes are contaminating the environment mainly because of its high organic load and sulfidecontent, as well as its content of inorganic salts of chloride, ammonia, chromium, and sulfate [2]. Pollution by tanneries did not receive enough attention in the Sudan. Large amount of un-reacted (untreated) WW from Gezira Tannery Corporation (GTC), Wad Medani, Central Sudan, is discharged into the nearby areas, viz. the natural forest east of the tannery, which lies between the tannery and the blue Nile and Atra village. Such Waste Water contains a percentage of chromium and this practice has resulted in a total destruction of the forest surrounding the tannery. Moreover, some of these effluents are suspected to drain into the Blue Nile, which is very close to the premises of the tannery.

The objective of the present work is to determine the level of chromium III in GTC wastewater and compare with the permissible limits.

The experiments were conducted in the Environment and Natural Resources Research Institute (ENNRI), National Center for Research (NCR), Khartoum, Sudan.

Sampling

Samples of Waste Water were collected from two locations: the outlet of the stream of Waste Water and the pond inside the previous forest which is used as an evaporation site(Figure 1). Samples from both locations were tested to determine the concentration of chromium.

Figure 1: (A) The outlet of the WW stream (Location A), (B) The forest (Location B).

The collected samples were acidified at the time of collection with nitric acid (2 ml of 69-72% concentration) to minimize precipitation and adsorption of the chromium cat ion on the walls of the container (100 ml). Time between collection of samples and analysis was about 24 h.

Waste Water (WW)

The determination of chromium-content in the WW was achieved by using the atomic absorption spectroscopic (AAS) method which is widely used in water analysis field [3].

Procedure

To a 100 ml sample, 2 ml nitric acid (HNO₃; 69-72%) and 5 ml HCl were added; then the solution was heated for 15 min on a hot plate, cooled slightly and filtered with suction through a sintered-glass into a clean flask. The filtrate was transferred to a 100 ml-volumetric flask. The solution was diluted to the 100 ml mark, mixed thoroughly and used directly for determination of chromium by atomic AAS model: 3110 (Perkin- Elmer Type). Data were taken directly from AAS.

Analysis of the data

The data were subjected to ANOVA analysis.

The results of chemical analysis of the WW collected from the two locations, i.e. LA and LB for the determination of the concentrations of chromium are presented in Table 1. The concentration (g of Cr/100 ml; i.e. %) obtained from location (LA) at the beginning of the stream (1.85%, i.e. 18500 ppm) was higher than that detected at location (LB), i.e. the forest (0.25%=2500 ppm; tail of stream). Both values are higher than the permissible limits (i.e. 0.0001%) according to Richard and Ivanildo [4].

Table 1: Concentration of chromium III (g/ml or %) in the WW of GTC, Wad Medani, Determined by AAS method.

The tanning industry uses some compounds that are known to be toxic. Therefore the waste/effluents of GTC are expected to be toxic, too. The indicators for such toxicity were the death of almost all trees of the forest closer to the GTC, where this WW is released for a very long period of time. GTC used to have water treatment unit during the 1970s, which is not functioning now for some reasons.

Chemical analysis for GTC wastewater using AAS demonstrated the presence of high quantities of chromium (1.85%). Their concentrations were more than the permissible limits (0.0001).

Wastewater from GTC and the other tanneries must be treated to remove all toxicants, and should not be allowed to be released untreated to bare land or a forest.

Currently, since the water treatment unit is not working, WW must be released into specially designed evaporation ponds; the remaining residues must be collected for final proper disposal.

Chromium must be substituted by another input because of its high toxicity and accumulation.