Flushing of Soils Highly Contaminated with Cd Using Various Washing Agents Derived from Sewage Sludge

Barbara K Klik,Zygmunt M Gusiatin,Dorota Kulikowska

doi:10.3390/en15010349

Abstract

The suitability of sewage-sludge derived washing agents (SS_WAs) (dissolved organic matter DOM; humic-like substances HLS; soluble humic substances SHS), was assessed for removing Cd from highly contaminated (300 mg/kg) sandy clay loam and clay. The soils were remediated via column flushing at two flow rates, 0.5 and 1.0 mL/min. The stability of the flow velocity (FV) depended on the type of SS_WA and decreased in the following order: DOM > HLS > SHS. Cd was most effectively removed during the first hours of flushing, and the process proceeded with a first-order kinetics. The overall process efficiency was higher at flow rate of 1.0 mL/min than at 0.5 mL/min and ranged from 65.7 (SHS) to 75.5% (DOM) for the sandy clay loam and from 64.7% (SHS) to 67.8% (DOM) for the clay. However, all SS_WAs at both flow rates removed the most mobile Cd fraction (F1) with an efficiency above 90%. Flushing improved soil characteristics in terms of the content of organic matter, humic substances and nutrients. Among all SS_WAs, DOM was the most suitable for remediation of highly Cd-contaminated soils due to high efficiency of Cd removal, the high stability of its FV during flushing and the simple manner of DOM recovery from sewage sludge.

Highlights

Cadmium (Cd) is one of the most mobile, bioavailable and toxic heavy metals (HMs) in soil
Cd was removed more efficiently from S1 than from S2. This was related to the properties of the soils, such as soil texture, organic matter content and sorptive capacity, which affected the strength of Cd bonding in the soil
The difference in Cd removal efficiency between sewage-sludge derived washing agents (SS_WAs) used were noted only at 0.5 mL/min: for both soils the process efficiency decreased in this order: dissolved organic matter (DOM) (67.6% for S1 and 64.3% for S2) > humic-like substances (HLS) (65.3% for S1 and 60.5% for S2) > soluble humic substances (SHS) (58.1% for S1 and 55.3% for S2)

Summary

Introduction

Cadmium (Cd) is one of the most mobile, bioavailable and toxic heavy metals (HMs) in soil. Toxicity and common co-occurrence with other soil contaminants, Cd constitutes a threat to environmental quality, food safety and human health [5,6], and it should be rapidly and permanently removed from soil. The treatment of soils contaminated with HMs is challenging task for environmental engineers because soil properties and metal concentrations differ greatly, even within small areas [7,8]. Soil washing is an ideal method for removing Cd due to the metal’s high mobility [3]. Even though a variety of washing solutions can effectively remove HMs, screening of appropriate washing agents (WAs) should include examine their efficiency (improvement of metal mobility and solubility), and their effect on soil quality. A WA should be nontoxic and biodegradable, interact weakly with the soil and allow the soil to retain its fertility [10]

Objectives

Methods

Results