Nickel (Ni) is widely
distributed in nature and is found in animals, plants, and soil; the
concentration of Ni in soil is approximately in the range of 4–80 ppm. Large
amount of Ni is released in the atmosphere due to natural as well as
anthropogenic activities including fossil fuel consumption, the industrial
production (mining, smelting, and refining), use, and disposal of nickel
compounds and alloys, and waste incineration. Human exposure to Ni results from
Ni contaminated food ingestion, water, inhalation, and percutaneous absorption.
According to International Agency for Research on Cancer evaluation, Ni
compounds are carcinogenic to humans and are classified as Group 1. Mixtures of
Ni metal and compounds cause cancers of the lung and of the nasal cavity and par
nasal sinuses. The maximum permissible concentration for Ni set by WHO in
drinking water is 0.07 mg/L, whereas National Standards for Drinking Water
Quality, Pakistan (NSDWQ-Pak), suggest the guideline value of 0.02 mg/L. The
concentration of Ni varies from <0.001–3.66 mg/L in ground water to
<0.001–1.52 mg/L in surface water in Pakistan. It was observed that, in most of
the cases, groundwater is contaminated with Ni beyond the contamination level
set by NSDWQ-Pak or WHO. Similarly, 75% of the surface water samples from the
largest city of the country (Karachi) exceed the limits. In a study from Lahore
(North and East zone), the wastewater samples were collected to evaluate the
waste water irrigation impact on vegetables. The Ni concentration was found to
be the highest than the reported studies from Pakistan and ranged between 0.91
and 5.94 mg/L and exceeded the permissible limit of 1.0 mg/L set by National
Environmental Quality Standards, Pakistan.
In soil, the highest concentration of Ni is 324 mg/kg (mean 172 mg/kg) from
contaminated Lahore site, while mean reference value of 70 mg/kg (Pb-Zn sulfide
horizon/mineralized site) in Kohistan region was found, which is far more than
the permissible limits set by EU or USA standards of soil on which sewage sludge
can be applied (30–75 and 210 mg/kg), respectively. This was attributed to the
dispersion of metals due to mining and may pose potential threats to local
communities of Kohistan region. Moreover, in another study conducted on soil of
Jahangir Valley, Punjab province, the maximum total content of Ni was recorded
as 81 mg/Kg (mean 31.93 mg/kg); the author concluded that these values do not
pose any potential health hazard to the general population. In coastal sediments
of the Arabian Sea along with the urban Karachi, the maximum concentration of
74 mg/kg Ni was found at the Lyari location at the most downstream part of the
Malir River. Similarly, in another study the second highest value of 56.46 mg/kg
was found at Karachi Port Trust (KPT) Boat Building Area. As per IARC, Ni
compounds are human carcinogens by inhalation exposure; therefore, no safe level
for nickel compounds can be recommended in air (assuming a linear
dose-response). In the current analysis, the concentration of Ni in particulate
matter was reported in the range of 0.001–0.15 μg/m3 and the highest of its
content was reported in urban atmosphere of Islamabad. In vegetables, the
concentrations of Ni ranging from <0.02 to 67.8 mg/kg with mean value of
30.1 mg/kg was observed in vegetables irrigated with sewage water in the suburbs
of Peshawar city, KPK. The author reported the significant positive correlation
of plant heavy metal with the given heavy metals in soil. In another research,
the second highest mean Ni concentration of 28 mg/kg was observed in Spinach
(Spinacia oleracea) irrigated by sewage water Hassan Abdal area, Punjab
province.