A Review of the Distribution Coefficient (K_d) of Some Selected Heavy Metals over the Last Decade (2012-2021)

This review synthesizes the methods for estimating the distribution coefficient (Kd) and provides a compilation of Kd values for five heavy metals (As, Pb, Cd, Cu, and Zn) based on research published in the last decade (2012-2021). We used the PRISMA method to ensure the transparency of the collected data. For mono-metal systems (MS), the Kd values ranged from 10-2 to 107 L/kg for Pb, from 10-2 to 106 L/kg for Cd, As, and Zn, and from 10-2 to 105 L/kg for Cu. In competitive systems (CS), the Kd values ranged from 10-2 to 105 L/kg for Cu, and 10-2 to 104 L/kg for Pb, Cd, and Zn, while no Kd value for As was reported under CS. It was found that the Kd values of heavy metals are affected not only by soil chemical and physical properties but also by the nature and characteristics of the metal involved along with experimental conditions. The totals references number of Kd data observation per element metal are represented as follows: Cd 35 (50%), Zn 35 (50%), Pb 33 (47.14%), Cu 33 (47.14%), and As 19 (27.14%). Overall, most research was done 1) on MS rather than CS, 2) on sorption rather than desorption, 3) on soil rather than sediments , and 4) most literature have reported the Kd values, derived from batch method than on column method. Despite significant progress over the past decade towards a better understanding of the variation in Kd values and the effect of factors influencing them to provide important parameters for predicting and controlling toxic metals in soils, additional research is still warranted to the complexity of underlying processes.