Modelling cadmium and lead uptake from irrigation water by some vegetables through transpiration in the semi-deciduous forest zone of Ghana

Heavy metal contamination of agricultural soils is a major concern to food production all over the world. Agricultural soil heavy metal contamination is from wastewater irrigation, application of sewage and air deposition from the atmosphere and is of great importance because of its implications for human health. Systemic health problems can develop as a result of excessive accumulation of dietary heavy metals such as Cd, Pb, Ni, Cu and Cr in the human body. A study was carried out at the Kwame Nkrumah University of Science and Technology (KNUST) in Ghana using water to which Cd and Pb had been added to irrigate cabbage, carrots and lettuce. Cadmium solutions of concentrations 0, 0.05 and 0.1 mg L-1 and Pb solutions of concentrations of 0, 30 and 50 mg L-1 were prepared and used to irrigate the crops. Kinetics of Cd and Pb concentrations in irrigation water were studied by preparing a 1:1 ratio of the sandy loam soil from the experimental site and irrigation water and shaken for 8 hours. The results showed that Cd concentration stabilized within the first 1 hour and the partition coefficients (Kd) for the 0.05 and 0.1mg L-1 irrigation water concentrations were 4 and 11.5, respectively. For Pb solutions the partition coefficients (Kd) were not constant but varied with time with values ranging between 0 and 6.94 after 6 hours of shaking. Moisture content of soil is known to influence the release of metals in soil solutions, however agricultural soils are generally of low moisture content. It was found from the study that the release of metals in soils was non-linear resulting in variation in the metal distribution coefficient. Soil Ca and dissolved organic carbon (DOC) concentrations gave a better prediction of soil Cd concentration than soil solution pH. Plant and soil samples from the experimental fields were collected for laboratory analysis. The results showed a reduction in the yields of lettuce by 11 and 16% for the treatments with Cd concentrations of 0.05 and 0.1 mg Cd L-1 in irrigation water respectively, compared with yields from the control treatment (0 mg Cd L-1 in irrigation water). On the other hand, there were increases of 61 and 53%, respectively in yields of carrots irrigated with water containing 0.05 and 0.1 mg Cd L-1. Yields of crops irrigated with water containing Pb concentrations of 30 and 50 mg L-1 were reduced compared with yields from the control plots suggesting that lead has the potential to suppress yields of crops. Plant Cd and Pb concentrations increased significantly with irrigation water concentrations of Cd and Pb with p-values of <0.0001 for Cd and <0.05 for Pb. Cadmium concentrations for cabbage were between 0.09 and 1.11 mg kg-1 while carrots and lettuce had values between 0.04 and 1.0 mg kg-1 and 0.12 and 1.02 mg kg-1, respectively. Lead concentrations in cabbage were between 0.18 and 15.2 mg kg-1 while for carrots and lettuce the concentrations were between 0.43 and 6.24 mg kg-1, and 1.41 and 187 mg kg-1, respectively. The percentage of the total Cd in the soil that is considered available, based on extraction with 0.01 M CaCl2, ranged between 18.7 and 96.8% for the top 5 cm depth and between 8.5 and 89.6% for the 5-10 cm depth. The percentage of soil total Pb considered available was in the range 0.058-7.86% for the top 5 cm depth; and 0.077 - 2.78% for the 5-10 cm depth. Both soil total and available Cd and Pb decreased with depth. Free ion activities of cadmium constituting the ions absorbed by the plants were determined by prediction using Windemere Humic Aqueous Model (WHAM) VI and measurement by cadmium electrode. Inputs for WHAM VI included soil properties like organic matter content in humic and fulvic forms, Na, Mg, K, balanced cations and anions. However, lead free ion activities were determined by prediction using WHAM VI only due to lack of lead electrode. Mathematical modelling of heavy metals uptake by plants is useful for predicting the quality of crops produced for human and other animals. The models also help to predict yields of crops produced using soil or irrigation water that is contaminated by heavy metals. In plant science, modelling of solute uptake is achieved by empirical models fitted with an equation or a set of equations to data. A modified transpiration model was therefore developed based on climatic variables such as temperature, saturation vapour-pressure deficit and relative humidity that govern transpiration and used to estimate crop dry matter content and irrigation water metal concentration, Cd and Pb concentrations in the test vegetables. The estimated and measured values were compared to determine the efficiency (EF) of the developed model. For Cd, EF values ranged between 0.911 and 0.953, while for Pb, the range was 0.514-0.995. The overall results showed that the model output using transpiration pull driven by environmental factors was reliable to predict the uptake of the heavy metals.

Source: KNUST